Out of the blue today the addons wouldn't show up in the configuration tab when I pressed "Get Extension Info." So of course I went into Febe and Verified my Extension Directory -- no such luck this time!
I even tried uninstalling the annoying addons that install themselves (such as DivX) and restarting -- still giving me the cold shoulder.
Then, by accident I found the easiest solution. Close Firefox and go into the Profiles folder. Open OPIEedits.json in a notepad. Copy all the text. Paste it into a new blank text file. Close OPIEEdits.json and rename it or move it. Save the new text file as your new OPIEedits.json in the Profiles folder.
Open Firefox and, Voila! OPIE should work now.
You can even try deleting OPIEedits.json all together, but you may lose your custom extension strings.
I think the encoding in the .json file gets messed up or something. So a simple remake of the same file in plain text gets it straight. Just a theory.