Man FF is annoying, they do a point release without adding any new features but still decide to have it break all the extensions. Anyways, the dictionary files are updated, but since mozilla is taking forever to allow the dictionary files to be added to their mozilla extension site allowing automatic update, you need to manually reinstall the dictionary files below to have Perapera work on FF5.0. The plugin itself should not need any kind of reinstall.
Rohan has spent years studying Japanese, Chinese and Korean, and currently lives in Japan. He created the perapera pop-up dictionary plugins to help other learners of Chinese and Japanese.