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.
- 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..."
- 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.
- 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.
- 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".
- Install your-file.xpi by opening it in the browser via File->Open. Follow the install steps Firefox presents.