Old extensions are abandoned by the author and tend to break on newer versions of the application. Other times, the extension is simple enough to work no matter what version you're running.
To manually modify the extension to work with the latest version, simply:
- Rename the extension file to a zip file.
- Extract the "install.rdf" file.
- Edit the "maxVersion" tag to match what you have (or more).
- Replace the old "install.rdf" file in the zip with your edited version.
- Rename it back to an xpi file.
- Install !