The Mozilla Firefox web browser has a really annoying safety feature where the version control system for Add-Ons (aka Extensions) won’t allow them to be installed on a higher version of Firefox than they’ve been tested on and approved for.

In theory this is a very good idea. The thought is that when new versions of Firefox come out, older versions of Add-Ons may not be compatible with the changes in the browser, and may then cause errors or system instability.

In reality most of the changes to the browser itself in dot releases (eg. from 3.0 to 3.1) are relatively benign and they rarely change anything related to most Add-Ons. This is not an issue if you only use add-ons created by proactive developers because they are generally quick to release compatible versions shortly after the browser update is released. But not all the time!

I have discovered two ways that you can get around this problem, and in my experience doing this over the past two years, I have never yet had an add-on that is marked as incompatible that didn’t work with the latest official and beta Firefox releases.

Here’s how you can do it yourself:

1. Install the Add-on Compatibility Reporter: “After installing this Add-On your incompatible extensions will become enabled for you to test whether they still work with the version of Firefox or Thunderbird that you’re using”. You can also report back to Mozilla whether there are any problems or not. This is a quick and easy way to make officially incompatible extensions work with your version of Firefox. I have only just started using this tool, so I don’t yet have any good data on how well it works.
2. Hack your Add-Ons: The Add-Ons available from the Mozilla Add-On Repository are simply container files (similar to .zip or .msi files) that contain a bunch of other files. You can use 7zip to open those containers, and then use a text editor to edit the “install.rdf” file contained within to change the version control information. Here’s a step by step tutorial:
   • Find your desired add-on from https://addons.mozilla.org/
   • Right click on the “Add to Firefox” link and save the file to your desktop
   • Install 7zip
   • Right click on the .xpi file you downloaded to your desktop, choose “7zip”, then choose “Open Archive”
   • Right click on the file “install.rdf” in the 7zip window, choose “Edit”
   • In your text editor search for some code that looks like “3.0.*“.
   • The number indicates the maximum version of Firefox that the add-on is officially compatible with. Edit this number to be something larger than the version you are trying to use it with. For example, if you are using Firefox 3.5, change this code to “3.5.*“
   • Save the file and close the 7zip window. Choose yes when you are asked if you want to update the archive.
   • Drag and Drop the .xpi file onto a Firefox window, and follow the prompts as required.

That’s it, you’re done. After restarting Firefox you should now have your favorite extension up and running again!

Note that while I have never had a problem in over two years of doing this, there is a risk that you could corrupt your installation of Firefox. You do so at your own risk and please don’t hate me if you have problems!

Good luck and be sure to let me know how it works out for you!