Sometimes you have an add-on for Firefox which you need badly but it is configured not to support newer versions of Firefox.
To change this behaviour:
Download the XPI file directly by right clicking on the install button and choosing “save target as…”.
Then unzip the file and edit install.rdf, change the version number to the actual Firefox version.
Example for JSview, change em:maxVersion
Don’t forget to save it afterwards. Zip the whole directory again and rename it from .zip to .xpi. Now you drag it to start installation process.
If the add-on was installed already directly change the file install.rdf in the according add-on folder.
Control the behaviour of the add-on afterwords in Firefox. It could be that the version limitations had their reasons.