As to the error itself, it's not Flagfox's fault. For some unknown reason, you're missing a very important Flagfox file that came in its installer. I see 3 possibilities here:
- Firefox botched the installation at some point. (Firefox does addon installations, not the addon)
- You have some kind of poorly written piece of software installed which deleted this file. (system cleaner, anti-virus, etc.)
- Your hard drive is corrupted in some way and the file was lost. (worst-case scenario; running a disk check is always a good idea)
Please reply back to tell me your results.