Easily Make Old Extensions Work with Current Version of Firefox

While Mozilla Firefox is an extremely customizable web browser, the speed at which new versions release can sometimes outpace the update speed for extensions.

This is hardly surprising; many extensions are developed by hobbyists whose sole job is not to work on the future of Mozilla’s browser.

You may find that an interesting extension does not work with the latest version of Firefox. However, compatibility can be forced: there is no guarantee it will work perfectly, but it can be tried as a last resort.

Forcing Version Compatibility

Begin by checking the version of Firefox you are using. The quickest way to do this is to press “Alt”, then select “Help” and “About Firefox”. This should open a window with the version number clearly visible.



Find the desired extension on Mozilla’s add-ons website.


Right click the “Download Now” button on the addons website, then click “Save Link As.” The window should give the file extension as an .xpi. If so, confirm the download.

Open the .xpi file using WinRAR or a similar file extraction program. Given that it is essentially a container for other files (and the .xpi format is used for all Firefox extensions), these programs should still be able to open it despite being unfamiliar.


Inside the .xpi there will be a file called “install.rdf.” Open this in Notepad, TextEdit, or an equivalent program. You may wish to use Word Wrap at this point – the line of text is likely to be very long.


Search within the .rdf file for the words “maxVersion.” This can be made faster using “Ctrl + F”. The number following this should be lower than your current version of Firefox, hence the issue with installing it normally.

Either change the value to your current version of Firefox or another higher number, and save the .rdf file. Naturally, changing it to a higher number will result in longer compatibility, but not necessarily functionality.

Double check the .rdf; as it’s in an archive, it may not have saved the change. If this is the case, extract the .rdf, edit it, and then replace it in the archive again. The “Date Edited” information can help determine if your tweaked version is in the archive.


Open the “Addons” menu within Firefox; this can be done quickly by pressing “Ctrl + Shift + A”. On the top right of the page, there should be a search bar and a button. Click the button, then select “Install Addon from File.”

Navigate to the .xpi file you modified, and double click it. Restart the browser and the addon should be installed. For addons that do not normally require you to restart the browser, you can skip this step.


While the process is a little more time consuming than simply downloading an extension, it can breathe new life into a project abandoned by its developer or allow you to regain functionality that may have been lost.


No guarantee can be made that the extension will work flawlessly. Generally themes are less likely to break due to changes than other extensions, as the Firefox UI is not as prone to change as back-end code that interacts with the extension.

Note also that due to Creative Commons licensing, you should avoid distributing your modified version of an existing extension: they should be solely for personal use unless otherwise stated.

Paul Ferson Paul Ferson

Paul is a Northern Irish tech enthusiast who can normally be found tinkering with Windows software or playing games.


  1. I’m disappointed that this did not work for the Fastest Fox add-on. The only number after the maxVersion that I saw was 18, which makes me wonder because the add-on worked right up until the latest FF update to V35.

    Sadly, others have said emails to the dev have bounced back, so this add-on may be pining for the fjords! :(

    1. Hi Steve,

      I’m just after testing this method with FastestFox, and was able to get it installed with 35.0. What archival tool are you using? I use WinRAR myself, so if you use something else that could be the point of contention.

      Try opening the install.rdf file again and changing “18.*” to a higher number. I changed it to the frankly ridiculous “218.*”, then saved the modified .rdf file. WinRAR prompted me to update the files in the archive as they had been changed. Afterwards I was able to install the add-on manually.

      To be extra-sure, change the .rdf file, save it and close Notepad. Open the file again and see if it has saved or not. If it hasn’t, extract the .rdf from the archive, to another folder, change it, then move it back into the archive.

      If that doesn’t work then let me know, and I’ll do my best to get to the bottom of this issue.

        1. Hi there,

          Have you double-checked the install.rdf file after you’ve edited it? It’s possible the .rdf file hasn’t updated if you change it within the archive, but if you’ve tried that we’ll have to try and figure something else out.

          What add-on are you trying to install? If you can’t get it to work I’ll try it as well.

          1. hey there, i’m having the same issues as steve. i double checked the changes by opening the file again and even went so far as to open the .xpi file with winzip (the one difference from the suggested method) running as an administrator to ensure that the changes would have the intended effect. the max version is set to 218, but it has not made any difference on compatibility, in my case. i do notice when i open the addons menu and click on the fastestfox options, there is a brief warning type window that appears and suddenly disappears again without any text on it (not that i cannot read the text, but the text even fails to appear). at this point, i select the options i want, but upon exiting the tab or restarting firefox, the selected options are not selected and the addon is not working as before. going back to the extensions folder in the firefox “appdata” directory (you know the one) there is a folder called trash that has my updated .xpi file in it and another copy that is not in the trash (still in the extensions folder) that also has my changes to it. hope this helps or drives a stake through the heart of the fastestfox extension, cause the worst is not knowing.

  2. “Note also that due to Creative Commons licensing, you should avoid distributing your modified version of an existing extension: they should be solely for personal use unless otherwise stated.”

    That’s a little misleading – the entire *point* of Creative Commons licensing is that you can redistribute it. The only exception would be CC-ND, which only allows you to distribute unmodified versions (but this license is rarely used).

    CC licenses are also not very common for software (not themes) to begin with – usually there will be software-specific licenses like the MIT, BSD or *GPL licenses.

  3. It Worked for me..for the extension Fireshot v.0.98.63. And Can Only be Installed in Firefox 33.0
    Everything Worked for me,just follow the instructions here…This Works!
    Thank You Very Much!!!

    Needed Tools: WinRAR,Notepad.

    1. Hi Barry,

      A pleasure to hear that this method has worked for you! Hopefully it will continue to work into the future, and Fireshot will retain compatibility.

      Thanks for your comment!
      – Paul.

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