I use French and English in almost all of my emails and this is a nightmare.

There is a (quite simple) solution, but you need to hack one of your dictionaries: copy-paste the content of e.g. your English dictionary into your French one; it works! As simple as that!

This is how I solved this issue on Mac OS X:

Go to /Users/HERE_YOUR_USER/Library/Application Support/Firefox/Profiles (use Cmd - Shift - G if your Library folder is hidden), and select your profile folder (mine was default.uo0 ) and the extensions folder. You will see folders for each of your dictionaries ( en-US@dictionaries.addons.mozilla.org and fr-classique-reforme1990@dictionaries.addons.mozilla.org in my case). In each main folder, a dictionaries folder. Copy the content of the .aff and .dic of one of your dictionaries into the .aff and .dic file of the other. Don't copy the first line in the .dic file; it's the word count!

Warning: in the .dic file, update the word count on the first line. It must be the number of lines of the file minus 1 (the first which is the word count).