Some time ago we wrote about how to make Firefox Add-on compatible with the latest Firefox version. The first method in that post doesn't seem to work, however using Nightly Tester Tools still does. But since then, an even better and lighter extension for this was released, which allows you to make any Firefox add-on compatible with any FF version, and also give back to the community by reporting if the extensions actually work with the Firefox version you are currently using (this is done with a simple click into the extensions dialog).
The extension I am talking about is called Add-on Compatibility Reporter and is very easy to use. A "Compatibility" option will be available for each of your installed extensions and all you have to do is click it, and select "This Add-on still works":
But how to install Firefox extensions for which the "Install" button is not available? For that, see the second part of THIS post. That still works perfectly.