HOW-TO: Force incompatible Firefox Add-ons to Install

If you ride Firefox versions harder and faster than add-on developers upgrade their add-ons for compatibility, this post is for you.

Firefox add-ons include their Firefox version compatibility within their .xpi install file. Often developers will put a "maxVersion" field in their add-on to mitigate potential compatibility issues between their add-on and future version of Firefox. 99% of the time however, there aren't any compatibility issues when there are minor (or even major) version upgrades of Firefox, yet you're prevented from installing your favorite add-on because of the add-on thinks it's incompatible. Here's how you can force the add-on to install. NOTE: your mileage may vary; you may indeed be installing an add-on that is not compatible with your up-version of Firefox; use at your own risk.

1. Download the .xpi for the add-on you want, directly to your hard-drive, bypassing the default installation behavior of Firefox. To do this, find the .xpi file for the add-on, generally through a Google search. addons.mozilla.org tries to be smart and prevent you from even downloading add-ons that aren't compatible with your browser, so you have to work around this and find the direct link to the .xpi. Once you find a link to the .xpi, right-click it and select "Save link as..."
2. Crack open the .xpi file. .xpi files are simple zip compressed archives, so you can unzip them like any other .zip file. "unzip your-file.xpi". That will un-archive/un-compress the contents of the .xpi file. You might want to do this in a dedicated directory for cleanliness' sake.
3. Open "install.rdf" (that was extracted in the previous step) in a text editor, modify the "maxVersion" field and save the file. Make sure this field is at least at the level of your version of Firefox. Use '*' accordingly to indicate any version number.
4. Re-generate the .xpi file with the updated install.rdf file. You do this by running the "zip" command like so "zip -f your-file.xpi".
5. Install your-file.xpi by opening it in the browser via File->Open. Follow the install steps Firefox presents.