Rated 5 out of 5 stars
If you are having problems with ForecastFox, exit Firefox. Then go to your Firefox profile directory and delete the forecastfox.sqlite file. Restart Firefox and ForecastFox should be working fine. You will probably need to update your personal preferences in ForecastFox options, though.
I have tried this on several computers running both OSX and Windows, and it's worked on all. (I don't know if this also fixes the non-English language problem, as I am not affected; but it certainly fixed everything else for me.)
Please note that, if you are installing ForecastFox from scratch on Firefox 35+, you may have to bump up the max version in the install.rdf.