Disable the preloading of Firefox autocomplete URLs

Martin Brinkmann
Jul 24, 2017
Updated • Jul 26, 2017

Mozilla plans to launch a new feature in Firefox soon which will preload autocomplete URLs in the Firefox browser when users type in the address bar.

Designed to give the connection to these sites a boost by doing some initial connecting before the user activates those links, some users may prefer to disable this feature for privacy purposes.

The tweak will preload URLs displayed by Firefox's autocomplete functionality. When a user types in Firefox's address bar, suggestions are displayed as the user types.

Update: Mozilla clarified that " Firefox will do the server DNS lookup and TCP and TLS handshake but not start sending or receiving HTTP data".

The best match is highlighted in the address bar of the browser while other suggestions are displayed in a list underneath it. If the best match is a web address -- URL -- Firefox will preload it as Mozilla sees this as a strong intent that the user wants to open that URL.

Firefox autocomplete URL preloading

firefox URL autocomplete

Firefox displays suggestions from search, the browser's bookmarks, and the browsing history by default when users type in the browser's address bar. Check out our Firefox location bar mastery guide for information on how to customize the output.

Tip: you can delete auto suggest entries in Firefox as well.

Mozilla highlights the process on the official Bug listing on Bugzilla@Mozilla.

When the awesomebar autocompletes to a URL, that is a strong signal that the user has an intent to visit that page. We have a hypothesis that we could speed up the perceived load time of pages if we start preloading pages in the background once we get that intent signal.

This could be particularly valuable, since a behavior that we see in user tests all the time is that users actually type out entire domains, even though it is getting autocompleted. For those users, we could make pages appear to load almost instantly. For users who accept autocomplete results faster, we'd still get a good speed gain in most cases.

Disable the preloading

firefox browser urlbar speculativeconnect enabled

Firefox users may disable the new preloading functionality. Some users may want to do so for privacy, as they may not want connections to be made before they actually connect to sites. Since a preload is always a guess on what a user may do next, connections may be made to sites that the user won't visit.

The site may have been visited in the past already though.

  1. Type about:config in the Firefox address bar and hit enter.
  2. Search for browser.urlbar.speculativeConnect.enabled.
  3. Double-click on the preference.

The two values the preference can  be set to are:

  • A value of True means the feature is enabled -- Firefox will preload autocomplete URLs.
  • A value of False means the feature is disabled -- Firefox won't preload autocomplete URLs

Closing Words

Mozilla continues to make improvements to the browser to make it faster. Good news for Firefox users is that it is possible to disable the feature, something that it often not possible in other browsers out there (especially Chromium-based browsers).

Now You: Do you find the preloading feature useful?

Disable the preloading of Firefox autocomplete URLs
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Disable the preloading of Firefox autocomplete URLs
Mozilla plans to launch a new feature in Firefox soon which will preload autocomplete URLs in the Firefox browser when users type in the address bar.
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  1. Chris said on July 26, 2017 at 7:14 pm

    This article is incorrect. This feature does not preload anything from the web server.

    As the bug report title and the about:config preference name say, this feature simply *speculatively connects*, which means Firefox will do the server DNS lookup and TCP and TLS handshake but not start sending or receiving HTTP data. The first bug comment suggests preloading, but the discussion evolves to just speculative server connections:


    Firefox’s speculative connect is like HTTP “preconnect” which, according to MDN: “Provides a hint to the browser suggesting that it open a connection to the linked web site in advance, without disclosing any private information or downloading any content, so that when the link is followed the linked content can be fetched more quickly.”


  2. TelV said on July 25, 2017 at 2:54 pm

    Presumably browser.urlbar.unifiedcomplete will still function to prevent the “Visit” link from appearing.

    How about this one though: browser.urlbar.userMadeSearchSuggestionsChoice currently set to “True”. That sounds like a tracker to me.

    1. Jed said on July 25, 2017 at 3:30 pm

      It’s not, it’s an option to show whether or not search suggestions appear in the address bar.

  3. ippophe said on July 25, 2017 at 2:50 am

    are you sure you got the about:config key right? please see https://bugzilla.mozilla.org/show_bug.cgi?id=1348275 — in 4 places (in referenced code), the preference is listed as:

    only in one place (a comment) is the version you list referenced (browser.urlbar.speculativeConnect.enabled)

    1. jofo said on July 25, 2017 at 3:48 pm

      I think browser code changes:


      are using browser.urlbar.speculativeConnect.enabled. But testing code changes:


      are using browser.urlbar.speculativeConnection.enabled. Which may be a bug.

  4. firedog said on July 24, 2017 at 7:58 pm

    Thanks for the heads up. I clear browser history each time Firefox is closed. So this would only prefetch stuff I have actively bookmarked, right? In that case I will keep it on.

    But I have disabled some of the other prefetch functionality in Firefox, by using the instructions here

  5. max said on July 24, 2017 at 5:56 pm

    How are they going to handle sites which set cookies upon visiting for stuff like displaying new postings since the previous real visit?

    1. Martin Brinkmann said on July 24, 2017 at 6:09 pm

      I’m not too deep into this but I think that the preloading won’t preload the full site but makes the initial handshake only. Can anyone provide additional details on this?

      1. TimH said on July 24, 2017 at 6:19 pm

        Will it execute JS? Will it allow cookies to be set?
        Will the feature be extended to mouse hover over url?

      2. Martin Brinkmann said on July 24, 2017 at 6:23 pm

        As I said, I have not looked into this to provide a technical answer. I don’t think it downloads or executes anything, but that it will do an initial connect to the site to speed this part of the connection process up.

  6. Idiot said on July 24, 2017 at 2:04 pm

    Thank you very much! I’ve just disabled this preference in about:config

  7. Ben said on July 24, 2017 at 11:56 am

    Thank you for mentioning that Martin.

  8. Tom Hawack said on July 24, 2017 at 11:55 am

    No, no and no. This is another example of what I’ve had the opportunity to mention concerning Mozilla’s priorities for Firefox : speed over privacy :

    “Designed to give the connection to these sites a boost by doing some initial connecting before the user activates those links, some users may prefer to disable this feature for privacy purposes.”

    I cannot understand this policy, legitimate and coherent with a browser such as Chrome, not at all on a privacy dedicated browser such as Firefox. Moreover an opt-out option : browser.urlbar.speculativeConnect.enabled // true by default!

    I’d love to have the person who decided this option, who ultimately chose to have it set to true by default, and ask : WHY?

    1. Clairvaux said on July 24, 2017 at 7:07 pm

      How is this an infrigement on privacy ? If you’re afraid someone will know you have connected to how.to.blow.things.up.and.kill.people.com, or to meet.nice.nazis.and.make.friends.de, why do you go there in the first place ?

      If Firefox is suggesting that to you, it’s because you’ve been there many times in the past, or it’s in your favorites. If that’s the case and you care about privacy, then you will have applied preventive measures before : browse in private mode, use a VPN, use Tor, wipe your history, not let Firefox remember anything, use different profiles for mundane and confidential activities…

      Also, nobody has ever been inconvenienced just for connecting to a questionable site. Except in very repressive regimes, maybe, possibly, but then it’s likely there would be many other censorhip measures in place, and you already know better than to go unprotected to embrace.the.opposition.and.say.bad.things.about.our.great.leader.cn.

      Now if things have been blown up, and the police has found your smartphone in a pool of blood, and they can prove you connected ten times to that site just a week before, then you might be in a bit of trouble, granted. But briefly exchanging a computer handshake before getting out of there, realising it’s a mistake ?

      1. Clairvaux said on July 25, 2017 at 12:37 am

        There’s a thing you might have missed, it’s called light-hearted humour. Apart from that, yes, I really meant what I wrote. Privacy and anonymity are primarily necessary for people who risk serious trouble to their life, health, property and liberty, while not doing anything that would be deemed illegal in genuinely free countries. Just posturing in principle about privacy might be fun, but it’s another kettle of fish.

      2. m said on October 3, 2019 at 4:41 pm

        Do people really still toss around the idea that privacy is for those that “have something to hide”?

      3. point said on July 25, 2017 at 12:03 am

        Straw man much with those nsa triggering keyword urls? No need to exaggerate to that level.

        People might bookmark direct download links, pages that are generated via io/db intensive lookups, de/activation links for iot devices, etc, which connected behind the scenes might have unintended consequences, while one is just typing a -similar- url and moz://a attempts the most fitting from the matches.

        Reading this article now, and it being seemingly opt-out, even privacy aware users might not remember when FF56 arrives in 6-8 weeks time to turn it off. Thus the concerns of privacy.

    2. Troppo said on July 24, 2017 at 6:25 pm

      The default is necessarily a balance between privacy, being competitive, and providing ease of use features for everyone.

      The gains from preloading are important and readily noticeable. People wouldn’t understand why they have to wait so long to read a page on Firefox but not on other browsers. When all browsers don’t “cheat” with preloading, people just blame the internet being that way. When all browsers do cheat except one, you don’t blame internet any more, even though it’s still the legit culprit, you blame Firefox.

      On the other hand the privacy issue with this feature is rather limited. Can you describe it, before going for instantaneous rejection ?

      If you want my opinion, I’m not even sure this feature should be disabled as part of the privacy checkbox thing that should come to a future Firefox near you. If there was a privacy slider a la Tor Browser, I would tie it to the strictest setting.

      I will personally disable the feature though, and I expect it to be part of Mozilla’s guide to disable all automatic connections.

    3. Jed said on July 24, 2017 at 3:29 pm

      I personally see it as an incredibly useful and helpful feature, and besides, it’s not very privacy infringing at all. It just preloads part of the page in the background. The same information would be sent just by visiting it anyway. People overreact with features such as this, when actively going to the same webpage would send the same, if not more information anyway.

      1. Jed said on July 25, 2017 at 1:00 pm

        I have most of the sites I visit either in history or in bookmarks, meaning it autocompletes a lot. This will mean it will load even faster with not really any downside.

      2. Nebulus said on July 25, 2017 at 2:05 am

        Jed, I’d like to know how exactly is this feature useful to you? Thanks.

  9. Clairvaux said on July 24, 2017 at 10:42 am

    Very good feature. And you can disable it, too. What’s not to like ? It took me a long time to use autocomplete, and even just to understand it. But it’s a heck of a time-saver for some sites you visit all the time.

  10. jupe said on July 24, 2017 at 10:35 am

    If you have autocomplete disabled already is it necessary to disable this too ? or because autocomplete is disabled then it would make this redundant?

    1. ams said on July 25, 2017 at 4:46 am

      that’s a great question

  11. 420 said on July 24, 2017 at 9:18 am

    I’m not sure who is in a bigger rush to lose all their customers, mozilla or microsoft. Microsoft is winning but mozilla is closing in fast, lol.

    1. The dude said on July 24, 2017 at 9:48 am

      That makes no sense. Doing no preloading of any sense when all your competitors do would translate to a death penalty over time.

      And it’s important to let users in control so they can disable the feature.

      Firefox does several kinds of preloading right now, all of which can be disabled, all of which I disabled, this is just one more. All of them are necessary because competitors do all of them too and time gained from proper preloading can be huge.

      1. Nebulus said on July 25, 2017 at 2:02 am

        Sometimes, doing something stupid just because your competitors do it is a bad idea. Mozilla seems to have forgotten this (I’m not talking about this particular issue, but about Mozilla’s attitude in general).

  12. Yuliya said on July 24, 2017 at 8:47 am

    I don’t have that prefrence in about:config. I assume it’s because I use ESR52?

    1. Martin Brinkmann said on July 24, 2017 at 8:50 am

      Yes that is the reason. I don’t think it is yet in Firefox Stable or ESR.

  13. happysurf said on July 24, 2017 at 8:21 am

    Thank you for the tip.

  14. Anonymous said on July 24, 2017 at 8:09 am

    Just seen what Mozilla have planned for FF56 too:
    I was thinking in november to try FF57 to give it a chance. Actually with FF54, I’ll stop the updates from now.

    1. Jed said on July 24, 2017 at 3:31 pm

      Bit of an overreaction. It can easily be disabled, therefore a bit of a non-issue. They’re attracting more users and adding useful features in, but leaving in the option to disable it. They care about their existing users, as technically they’re under no obligation to provide an option to disable it.

    2. Anonymous said on July 24, 2017 at 9:30 am

      No man! Go back to Firefox 3! Trust me. Fking idiot /sacrasm

      1. Anonymous said on December 29, 2017 at 8:09 am

        still use ff3, except mostly on pale moon, because, yes, ff>3 pretty much is crap.

      2. bloatware hater said on September 17, 2017 at 6:29 pm

        I ran firefox three forever, then moved to palemoon because the current firefox sucks. The mo-silly quest for the perfect browser has resulted in the worst piece of software I’ve ever had to hack.

  15. ShintoPlasm said on July 24, 2017 at 7:34 am

    It’s possible to disable this feature in Opera, by the way.

  16. Nik said on July 24, 2017 at 7:13 am

    But which one will be pre loaded? Like in your example image, you typed mozilla.org. There are 10 suggestions listed and 1st is highlighted. But what if I don’t want to go to the 1st link. That seems waste of my bandwidth.

    1. Martin Brinkmann said on July 24, 2017 at 7:31 am

      The one that is shown in the address bar.

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