Is it listed in the add-ons manager? If it is, does it have any error message next to it in the add-ons manager? One possible cause for add-ons not working is one of the files that stores details of installed add-ons is corrupt. Try the following, open the Firefox profile folder, details of how to access this is shown in the [[profiles]] article. Once in the profile folder, close Firefox and then delete extensions.cache, extensions.ini, extensions,rdf and compatibility.ini. When you re-start Firefox those files will be re-created and in the process it will re-build the list of installed extensions and themes.