Install Firefox 3.6 add-ons in Firefox 4 beta, disable compatibility check

Posted on June 30, 2010

Update (March 22, 2011): this post also applies to Firefox 4 final. Just use extensions.checkCompatibility.4.0 instead of extensions.checkCompatibility.4.0b.

Firefox 4 beta is available. I installed the 64bit version on Linux today, it’s notably faster than previous versions and includes several other improvements such as tabs on top. Now, I don’t want to go back to 3.6.

As usual the only thing holding me back from upgrading is add-on compatibility. Luckily it’s possible to disable the compatibility check so Firefox 4 will let you install add-ons built for older versions, I found that many add-ons work just fine in Firefox 4.

Disable compatibility check

If you’re willing to take your chances, navigate to about:config in your browser, promise not to break anything and right to window click to add a new boolean. In the preference name field, add extensions.checkCompatibility.4.0b and set the value to false. Firefox will now let you install any add-on.

If anything breaks, try disabling add-ons or delete them completely. You can always go back to 3.6 if nothing works, and wait for a new stable release.

I’ve tested it with AdBlock Plus, Firebug, Flashblock and Wappalyzer.

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Comments (5)

  • thanks man

    Posted by :) on October 13, 2010 Reply

  • Hey there!
    I’ll give it a try. :)

    Btw: I just entered the deepweb.. This is my first post in here. ;)

    Posted by :] on December 02, 2010 Reply

  • That seemed to work for every add-on except FireFTP. It comes up, but has no connect button or add new.

    Posted by Wayne on January 28, 2011 Reply

  • Oops! It broke my Firefox 4beta11 installation!
    I think the culprit was Tab Mix Plus ( (at least partly) I couldn’t get to the Tab Mix Plus options… an error message appears in a big box with a few (4 lines?) lines of code. (sorry wasn’t able to get a screencap of that). I couldn’t access any website, can’t get to about:config… etc. In short, it was rendered useless.

    ABOVE: my Firefox after the tweak.

    ABOVE: a few of my addons.

    Posted by Lorgen Shadouness on February 25, 2011 Reply

  • Be sure to disable Tab Mix Plus with Firefox 4 until it’s compatible [Why? See my earlier comment/post]. I was able to disable it… (after some backups, uninstallation, reinstallation, copying… et al.) and things are going well again! =’)
    [Memory use though, is probably as high again :’/]

    (However, now that I can use Firefox 4 here (with the addons enabled) I can’t drag and drop images here for posting :’o)

    Posted by Lorgen Shadouness on February 25, 2011 Reply

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