Temporary Fix Rated 4 out of 5 stars
I'm also asking the developer to please update the add-on to work with Firefox 16+
But, there's a way to get it to work for now for the rest of us...
1. Launch Firefox
2. Weep at the lack of Forecastfox
3. Hit Ctrl+N to spawn a new Firefox window
4. Holy crap- Forecastfox is there!
5. Close original Firefox window that is missing Forecastfox
6. Go about your usual business