You have to update your extension's manifest every time a new version of Firefox is out. It is possible that your extension's maintainers haven't included Firefox 4 in their list of whitelisted versions even though it may work. Provided you are sufficiently technical and wouldn't mind dealing with crashes, install Nightly Tester Tools (now renamed to something more officially iirc, but searching for Nightly Tester Tools should get you there) and try to disable the compatibility check and see if it works. Back up your profile first so you can just copy it over again if something breaks horrifically.