I use French and English in almost all of my e-mails and this is a nightmare...
There is a (quite simple) solution, but you need to hack one of your dictionary: copy paste the content of e.g. your English dictionary into your French one, I works! As simple as that!
This is how I solved this issue on Mac OS X:
1) Go to /Users/HERE_YOUR_USER/Library/Application Support/Firefox/Profiles (use cmd-shift-g if your Library folder is hidden) then select your profile folder (mine was default.uo0) then the extensions folder.
2) You will see folders for each of your dictionaries (en-US@dictionaries.addons.mozilla.org and email@example.com in my case). In each main folder, a "dictionaries" folder.
3) Copy the content of the .aff and .dic of one of your dictionary 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...).
4) Restart Firefox! and now... vous pouvez taper en français et in English in the same text area!