Thank you Erikas, but I suggest you to use real upstream instead of this as the source of your package. Check it out by visiting https://launchpad.net/ispell-lt .

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.