Display the use of iframes in Firefox

Martin Brinkmann
Jun 1, 2018
Firefox, Firefox add-ons

Check iframe is a free open source browser extension for Firefox that reveals to you if websites use iframes and highlights the iframe code and the actual iframe on the website.

Websites may use iframes to display other HTML documents in a frame. It is often used by advertising scripts, social media scripts or media embeds.

While iframes are certainly useful in some contexts, they may cause privacy issues and concerns as well. Companies may read previously set cookies for example and they may gain access to data such as a device's IP address, web browser, and other information.

You can check the source code of any website to find out about iframe use. Tap on F12 to display the Developer Tools of the browser that you use and switch to the source code listing there (in Firefox that is Inspector). You can use the search to find instances of iframe. It works, but it is time consuming if you want to do so for multiple sites or even all sites that you visit.

Check iframe

Check iframe simplifies the process. The extension adds an icon to Firefox's main toolbar when you install it that uses color codes to highlight the use of iframes on webpages that you open in the web browser.

The extension supports for different color codes currently:

  • Blue icon -- not checked.
  • Green icon -- no iframes detected.
  • Orange icon -- at least one iframe detected.
  • Purple icon -- iframe detected and at least one matches a custom source added by the user of the extension.

The extension uses the JavaScript function document.getElementsByTagName() to detect iframe tags on websites.

A click on the icon displays options to run another scan for iframes on the page, scroll to the next detected iframe element and highlight it by drawing a red border around it, display the source of all tags found, or open the built-in configuration.

The configuration offers two options: you can enable log output to the console in Firefox and add custom sources to the extension so that you are notified whenever the source, e.g. an iframe by Facebook, is found on a page.

Closing Words

Check iframe is a useful privacy and maybe also development extension. It highlights when pages on websites use iframes, displays the code of the iframes, and highlights them on the page. It provides you with the means to spot small iframes used for tracking, and any iframe a site uses.

I would like to see an option to highlight all iframes on a page at the same time. The extension highlights only one at a time right now.

Now You: Do you analyse the code of sites you visit?

Related articles

software image
Author Rating
4.5 based on 2 votes
Software Name
Check iframe
Software Category
Landing Page

Tutorials & Tips

Previous Post: «
Next Post: «


  1. bawldiggle said on June 4, 2018 at 5:38 am

    Which version of FFox is this blog about … pre-Australis, Australis or Quantum ?

    1. Martin Brinkmann said on June 4, 2018 at 5:41 am

      The extension works in Firefox 48 and newer.

  2. Steve said on June 4, 2018 at 12:04 am

    @JD Thanks so much for the iframe settings in both the userContent.css and about:config files. I am very glad to have control over this important setting once again. I will also take a peek in nsPermissionManager.cpp to find other wholesome FF goodness.

    PS: I too use an older version of FF due to my fav addons not being ported to Quantum.

  3. JD said on June 3, 2018 at 10:04 am

    Sounds good, but for now will stick with goold old basic Gecko methods.

    1) Showing a colored border around all iFrames, by adding such a line in userContent.css or adblock.css:
    iframe { border: dotted 2px cyan ! important; }

    2) Pref to control frames+iframes in about:config
    permissions.default.subdocument (INT)
    0/1 = allow all / 2=block / 3=same domain
    This works at least since FF2, if users just create this pref manually on about:config

    There are lots more types available for this extremely handy permissions-family (permissions.default.xxx), as listed in nsPermissionManager.cpp. For global control of stylesheets, script files, XHR, media (html5-video), embedded plugin stuff, etc.
    Yes Mozilla has always tried to hide those prefs from users, very successfully. But nevertheless they work great since many years, if users just create those prefs manually.
    Am not quite uptodate to the latest FF-versions, but sure hope it’s still possible.

    An xpi-addon for handling the global prefs is “Block Content”
    An xpi-addon for site-exceptions is ExExceptions.
    Both addons handly only some types not all, but manually toggling other global types is easy.

    1. Sanjay Nayak said on June 8, 2018 at 6:19 am

      JD – can you share the URLs for Block Content and ExExceptions addons? Thank you.

  4. Steve said on June 2, 2018 at 4:02 pm

    Years ago users could block iframes via firefox’s about:config. That is, until shortsighted mozilla decided no one should ever use this valuable option and thus, removed it. Truth be told, I have no idea why I still use FF; the shortsighted lunacy FF users have to put up.

  5. noe said on June 2, 2018 at 7:54 am

    .. almost everyone already knows .. good old umatrix is superior; block 1st party & 3rd party iframes .. amongst other things. https://github.com/gorhill/uMatrix .

    1. owl said on June 2, 2018 at 8:43 am

      “UMatrix” is my favorite item. This extension allows you to fully control the object.
      On the other hand, this “Check iframe” is useful as a means to gather information, not to manage objects.

      In other words, a notification with good visibility is displayed on “icon”, so you can easily distinguish iframes.
      If necessary, you can easily obtain detailed information on iframes.
      “UMatrix” is sufficient for the control function.
      However, I felt it convenient to obtain information easily by “Check iframe”.

  6. owl said on June 2, 2018 at 12:37 am

    Thanks Martin! Great info.
    Immediately, I installed it and tried it.
    I felt it useful. I will continue to use for a while.

  7. Millenicide said on June 1, 2018 at 7:38 pm

    Would this addon help make it easier to write custom ublock origin filters? I find it difficult at times to find the frame I’m trying to write a rule for in a generalized way.

Leave a Reply

Check the box to consent to your data being stored in line with the guidelines set out in our privacy policy

We love comments and welcome thoughtful and civilized discussion. Rudeness and personal attacks will not be tolerated. Please stay on-topic.
Please note that your comment may not appear immediately after you post it.