Firefox Checking Update Compatibly Every Time? Try This Fix

Martin Brinkmann
Dec 28, 2011
Updated • Mar 11, 2015

Firefox since yesterday was checking for add-on compatibility on every browser start. It displayed the window that you only see after upgrading to a newer or downgrading to an older version of the browser.

I first though that this was caused by the excellent sandboxing software Sandboxie, but this turned out to be not the case. I tested it by disabling Sandboxie before running the browser. The add-on update check was still displayed every time I started the browser.

I then checked if it was an issue with the prefs.js or another file that does not get updated properly forcing the browser to repeat the same process on every start.

While I was looking through the Firefox profile folder I noticed a user.js file in there. And that file, according to Mozilla does not exist by default. The Firefox developers note that that "once an entry for a preference setting exists in the user.js file, any change you make to that setting in the options and preference dialogs or via about:config will be lost when you restart your Mozilla application because the user.js entry will override it".

This for my situation meant that the user.js file was blocking Firefox from updating properly. I have no idea how and why it was created in the first place, but decided to create a backup and delete it to see if it resolves the issue.

I closed Firefox first, copied user.js from the profile folder to the Windows desktop, and deleted the file afterwards. Firefox on next start did not check for add-on compatibilities anymore and the browser seems to have returned to its former self.

It is also interesting to note that the user.js file has not been created again by the browser.

Hope this helps Firefox users who are encountering the same issue that I did. I suggest you create backups of the file just in case for an option to restore it if this does not resolve the issue you are experiencing.


Tutorials & Tips

Previous Post: «
Next Post: «


  1. James Hanley said on August 15, 2012 at 12:34 pm

    Thanks a million for your tip. I’ve removed the JS file and the problem has now gone away. I was so tired of this annoying bug that I was actually thinking of changing over to Internet Explorer. Now, that is desperation for me.

  2. Edgar Nuum said on June 8, 2012 at 6:04 am

    Thank you for tip!
    Problem solved by removing user.js!

  3. gus said on April 2, 2012 at 5:43 am

    Thank you very much I thought it was a bug in version 11 =)

  4. ash said on February 19, 2012 at 9:25 am

    I got a problem like you, my firefox always check a compatibility for add on every time I started it… but I use your tips and my problem solved… thanks…..

  5. Mike said on February 18, 2012 at 5:13 am

    Sweet! Thanks a bunch I was having this problem for a bit and it would also keep blocking from entering sites saying they were not protected when their sites i visit all the time (Facebook, TD, CIBC)

    Thanks Again :)

  6. Midnight said on December 28, 2011 at 6:17 pm

    Great tip, Martin. Good to know!
    Thanks for sharing!

  7. Robert Palmar said on December 28, 2011 at 6:04 pm

    The user.js is not just user created but created by some software.
    If the software is still on the system the user.js will be recreated
    once again even if deleted with every new start of Firefox.

    Perhaps the software that created it on your system has
    since been removed since the user.js was not recreated.

    1. Transcontinental said on December 28, 2011 at 10:39 pm

      I’m wondering as well what sort of software can create a user.js file, this is out of the protocol!. I’m not an expert (need I mention) but frankly I’ve never heard such a thing before. Anyway, if you already have a user.js, would that software overwrite yours with its own ?

    2. Martin Brinkmann said on December 28, 2011 at 7:46 pm

      Wonder what kind of software this was.

      1. Robert Palmar said on December 29, 2011 at 12:52 am
  8. mithun.mi6 said on December 28, 2011 at 4:49 pm

    Hi Martin,
    I have the Tab app as an addon for my browser.
    But yesterday onwards I cannot access it for some reason . When I checked the addon settings it was still active. What could be the reason???

    1. Martin Brinkmann said on December 28, 2011 at 5:23 pm

      Did Firefox update?

      1. Anonymous said on January 4, 2012 at 3:19 am

        Sry for the delay.
        Yea it updated , but it is not showing any message about being incompatible with this version or anything like that.

  9. DanTe said on December 28, 2011 at 3:12 pm

    I had this happen to me occasionally at certain locations. I’ve always assumed that it was due to the local IT’s stupidity in implementation. Now I Know that it IS due to their implementation :)

  10. Threshold said on December 28, 2011 at 1:53 pm

    Martin is there a way to never have that update compatibility check?
    I hate it and having all versions of Ffox installed I get that a lot.

    Also in my profile folder I have Invalidprefs.js: is this supposed to be there and if those preferences are invalid how do I correct them?


    1. Martin Brinkmann said on December 28, 2011 at 2:58 pm
  11. Transcontinental said on December 28, 2011 at 11:53 am

    As far as I’ve always known, user.js is a user initiated file, it is neither part of the Firefox install neither created by Firefox during session, so the user.js you had in your profile must have been created by a user …
    I use user.js to either change or confirm by rewriting any changes which could have been brought to Firefox without my consent, but this is delicate if for instance you have modified manually a setting in about:config or if you have authorized an add-on to modify what could be then again counter-modified by your user.js … one has to stay aware of the pros and cons of user.js, obviously !

    1. Martin Brinkmann said on December 28, 2011 at 12:36 pm

      I’m just wondering how I got it ;)

      1. Transcontinental said on December 28, 2011 at 10:31 pm

        Or maybe Martin is kind enough to consider his system as an example, rather than explaining that it could have been, but was not really, but you see the story needs an example …)
        I had a teacher like that, he wished so much to get understood that he’d put himself in the explanation. It means you really like getting things to be understood..

      2. Midnight said on December 28, 2011 at 6:20 pm

        Perhaps someone gave it to you as a Christmas present! :)

  12. ódio said on December 28, 2011 at 11:46 am

    this happens with me only in Nightly

    1. Midnight said on December 28, 2011 at 6:19 pm

      As it’s suppose to.

      I’m using Firefox UX, installed on December 24th and noticed that the Nightly hasn’t been updated since.

      The developers must have taken a really long Holiday! :)

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.