How-To Geek

Hack Extension Files to Make Them Version-Compatible for Firefox

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A well known drawback in using Firefox is the problem with extension compatibility when a new major version is released. Whether it is for a new extension that you are trying for the first time or an old favorite we have a way to get those extensions working for you again.

There are multiple reasons why you might want to choose this method to fix a non-compatible extension:

  • You are uncomfortable with tweaking the “about:config” settings
  • You prefer to maintain the original “about:config” settings in a pristine state and like having compatibility checking active
  • You are looking to gain some “geek cred”

Keep in mind that most extensions will work perfectly well with a new version of Firefox and simply have the “version compatibility number” problem. But once in a while there may be one that needs to have some work done on it by the extension’s author.

The Problem

Here is a perfect example of everyone’s least favorite “extension message”. This is the last thing that you need when all that you want is for your favorite extension (or a new one) to work on a fresh clean install.

Note: This works nicely to “replace” non-compatible extensions already present in your browser if you are simply upgrading.


Hacking the XPI File

For this procedure you will need to manually download the extension to your hard-drive (right click on the extension’s “Install Button” and select “Save As”). Once you have done that you are ready to start hacking the extension.


For our example we chose the “GCal Popup Extension”. The best thing to do is place the extension in a new folder (i.e. the Desktop or other convenient location) then unzip it just the same way that you would with any regular zip file.

Note: HTG reader Dante pointed out that if you do not have a third party zip utility such as IZArc or 7-Zip installed changing the “.xpi” file extension to “.zip” will allow you to use Windows integrated zip software to unzip the file.

Once it is unzipped you will see the various folders and files that were in the “xpi file” (we had four files here but depending on the extension the number may vary). There is only one file that you need to focus on…the “install.rdf” file.

Note: At this point you should move the original extension file to a different location (i.e. outside of the folder) so that it is no longer present.


Open the file in “Notepad” so that you can change the number for the “maxVersion”. Here the number is listed as “3.5.*” but we needed to make it higher…


Replacing the “5” with a “7” is all that we needed to do. Once you have entered your new “maxVersion” number save the file.


At this point you will need to re-zip all of the files back into a single file. Make certain that you “create” a file with the “.zip file extension” otherwise this will not work.


Once you have the new zip file created you will need to rename the entire file including the “file extension”. For our example we copied and pasted the original extension name.


Once you have changed the name click outside of the “text area”. You will see a small message window like this asking for confirmation…click “Yes” to finish the process.


Now your modified/updated extension is ready to install.


Drag the extension into your browser to install it and watch that wonderful “Restart to complete the installation.” message appear.


As soon as your browser starts you can check the “Add-ons Manager Window” and see the version compatibility numbers for the extension. Looking very very nice!


And just like that your extension should be up and running without any problems.



If you are looking to try something new, gain some geek cred, or just want to keep your Firefox install as close to the original condition as possible this method should get those extensions working nicely for you again.

Akemi Iwaya is a devoted Mozilla Firefox user who enjoys working with multiple browsers and occasionally dabbling with Linux. She also loves reading fantasy and sci-fi stories as well as playing "old school" role-playing games. You can visit her on Twitter and .

  • Published 03/15/10

Comments (15)

  1. Michael Gallaher

    If you are a little less ambitious you can install the Nightly Tester Tools extension. It will let you override the compatibility issues with one or all of your extensions.

  2. johncalvin

    Maybe I’m missing something here when I “Save as” I get a .xpi file that I can not open or unzip. Any pointers?

  3. Bobbylight

    Would this work for Thunderbird extension’s?

  4. Enzer Milliard

    Is there any benefit in using this over Nightly Tester Tools ( ) to force extension compatibility?

  5. Asian Angel

    @Bobbylight – It should work just as well for Thunderbird extensions. ^__^

  6. Asian Angel

    @johncalvin – Which file zipping utility are you using? I have IZArc installed here and it unzipped the xpi file without any problems.

  7. Asian Angel

    @Enzer Milliard – While the Nightly Tester Tools extension works well this gives you another option for making extensions compatible and lets you have fun digging into some of the “inner workings” too. ^__^

  8. Asian Angel

    @Michael Gallaher – The Mr Tech Toolkit has always been a favourite of mine for forcing compatibility and getting other nice functions added to Firefox at the same time. ^__^

  9. Dante

    @johncalvin: You have to rename the file from “extension.xpi” to “”. Then you can unzip and proceed.

    On another note: once you’ve changed the install.rdf file, just drag it onto your “” folder and answer “yes” to the Do-you-want-to-replace-it-question. This substitutes the old file with your modified one. Then rename again from “” to “extension.xpi” and drag it into your open Firefox window.
    Saves you at least 3 seconds! ;-)

  10. Asian Angel

    @Dante – I was actually able to unzip the xpi file directly without any problems using IZArc. ^__^

  11. Dante

    @Asian Angel: Sure that works, but some people don’t have third-party software (like 7-zip or IZArc) installed. If you rename the extension, you can use Windows integrated zip software.

    Whatever works for you and makes your life easier!

  12. Asian Angel

    @Dante – Always good to learn something new. ^__^ Also adding a note to the article about the Windows integrated zip software. ^__^

  13. SquareWheel

    I prefer just setting my Gecko software (Firefox or Songbird usually) to accept outdated extensions. Unless you get annoyed by the warning in the Addons menu, then this is much simpler.

  14. Aerik

    Why don’t you just use the extension “nightly tester tools” ?? Much quicker, easier, and cleaner.

  15. Kristina

    It worked!! Thanks! Now I can use LongURLplease! You are awesome dude! I’m bookmarking your website!

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