How-To Geek

How To Bypass Firefox 4.0 Beta’s Incompatible Add-on Error and Install Extensions Anyway

Note: This article is part of our archive and is likely out of date.
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The latest Firefox Beta is rather awesome, with tabs on top, and a new menu that looks like an awful lot like Opera. It’s all very slick, until you try and install your favorite extensions that haven’t updated yet. Here’s how to force them to work.

Here’s the error message we’re talking about:


Side note: want to test the new beta out for yourself? Head over to the Firefox Beta site, though it’s really in beta, and has a lot of changes, so you’ll want to think carefully before you do so.


Open up the about:config page, and then right-click anywhere and choose New –> Boolean.


You’ll get a popup menu, at which point you’ll want to use one of the two values as the preference name, depending on the version you’re using:

4.0 Beta: extensions.checkCompatibility.4.0b

4.0 Release: extensions.checkCompatibility.4.0


Then set the value to false.


At this point your extensions should install just fine…


Important Note: At the moment of the writing of this article, Greasemonkey is really actually incompatible with Firefox 4.0 beta, and continually opens up the homepage with a never-ending list of tabs. The same goes for quite a few extensions that we tested, though some of them worked fine. We’re pretty sure they’ll be updated very soon.

Lowell Heddings, better known online as the How-To Geek, spends all his free time bringing you fresh geekery on a daily basis. You can follow him on if you'd like.

  • Published 07/7/10

Comments (22)

  1. SquareWheel

    Frankly, I wish Mozilla had just stuck with extensions.checkCompatibility.

  2. BobbyPhoenix

    Very nice both the Beta, and the way to make extensions work. All mine now work again. I thought some would need updating by devs, but they all work.

  3. Antrikshy

    Or… you could install this add-on here:

  4. Palmin

    Yes, using the Add-On Compatibility Reporter – is the recommended way (it even says so on the FF 4 Release Notes page), since you can directly give feedback about which add-on works and which does not.

  5. Palmin

    Yes, using Add-On Compatibility Reporter – is the recommended way, since it allows you to give feedback about if the add-on works or not.

  6. Santo

    Can these steps be tried for incompatible add-ons in Firefox 3.6.6?

    Tab kit 0.5.8 is not compatible.

  7. SquareWheel

    There is no sense in using an addon when you can do it manually. Why would you allocate your resources to unnecessary software?

  8. Mysti

    In addition to allocating unnecessary resources, I get a message from this addon saying “This addon no longer works.” Much better to do it manually.

  9. Sanchit Goyal

    Just a tip for users using 3.6.6 and wanting to try this trick.

    While creating the entry put the version as 3.6 and NOT 3.6.6 to make it work.

    I tried with 3.6.6 but it didnt work. Then I put it as 3.6 and it worked perfectly!

  10. sadirdin

    thanks very much for sharing this article
    also i try above that trick

  11. Vineen

    perfect article. i think i would share this on my twitter, fb and yahoo so everyone who want to have their add-ons not working on their new firefox v4.0 b7 would work out.

    this is nice! i’ve come an article. Facebook #Buffer will now work. :)

  12. che

    I’m not sure it’s working on 4.0b7

  13. Riston

    It gives an error in firefox 4.0b7

  14. Ryan Isra

    Great, it makes my FF4 hang up. Now typing this from –safe-mode

  15. Shino Shibu

    yeah it worked for me thank u very much

  16. Ofer Bezalel

    For the RC version use extensions.checkCompatibility.4.0 value, and it works great

    too bad that Tab Kit is only semi functional :-( (can’t set the tabs to align to the right side and have the tab-tree functionality)

  17. Ofer Bezalel

    If anyone is interested, with this hack on FF4 RC Tab Kit + Tree Style Tab get me very close to the full functionality I needed – now if only bartab were to work correctly, I would be a happy camper!

  18. joseph

    Mozilla 4.0 seems to have lot of features as i am a basic user of chrome and IE i would like to check out the new version of firefox.

  19. Lindsay

    This worked perfectly on the recently shipped version of FF4. I had to redo my toolbar customization (to add the Bookmarks and Tag buttons, not to show the whole Delicious toolbar, because one of the glories of FF4 is more screen real estate and fewer toolbars), but it is still pinning or automatically reclosing just like it used to. Thanks for the tip!

  20. PG

    Hi, I tried the tip for Firefox 4.0 to make my add-ons compatible but it didn’t work. The add-ons highlight and everything but they aren’t working right till they are not working at all. Can you please tell me what I did wrong?

  21. Works

    Even with FF 5 this works when you change the numeral in the boolean string from 4 to 5. For w/e reason, I had to restart the browser twice, then go into add-ons and enable one last add on that wasn’t auto enabling and everything is working as it is supposed to. THANK YOU!

  22. Works


    In 5, adding this extension means it leaves a permanent notice and an enable button at the top of your add on page for enabling checking compatibility again. If you ever accidentally click that enable button, you’ll have to go back into about:config and set it to false again and restart.

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