A very very useful plugin, works excellent in both Windows and Ubuntu, and in all versions of Firefox (that i have used so far).
One additional feature I am dreaming about is ability to automatically detect language. It's quite often a case when i write in Lithuanian in one tab, and then switch back to English. Constantly selecting spell checking plugin is quite a nuisance. I believe this feature request targets more whole Firefox app, than just this plugin.
Yep, what you want should be the app's functionality, not that of a dictionary (which I'm providing here mainly for the users of English versions of Firefox in Lithuania). Although, automatic language detection is not that easy, I guess. I think you may find it easier to develop a habit to use →K→A and →K→L to quickly change the dictionary in use without moving your hands away from the keyboard.
Though it's probably worth to file a feature request (if there isn't one) asking for Firefox to automatically choose a dictionary depending on the keyboard layout in use. It wouldn't cover all cases, but would at least be helpful in some.