It is the same every time a new version of Firefox is released: plugin authors need to check whether their extensions work with the new version, and then create a new package that includes the flags to say it indeed works.
So far, so good. But the next step, after uploading the package to the add-ons site, isn’t working very well.
A few days after FF 3.5 was released, I uploaded Define Word 0.9.2, to make it easy to get it to work in the new browser version. Today, the extension is still not approved, and anyone that used the extension in the past have probably given up by now.
If you haven’t yet given up, feel free to download it from here. Eventually, you may even be able to get it from Mozilla, but don’t hold your breath 🙂