Rated 3 out of 5 stars
Works great, but have some issues.
Sometimes it shows wrong language (different is chosen for spellchecking).
Sometimes detects wrong language without a reason and don't want to correct it even after putting a lot of text.
Manually changing language always locks the choice for that site.
Auto-detecting sometimes stops firefox responding for few seconds.
It should have icon that will change depending on chosen language.
Toolbar button is inconvenient, looks ugly (like it is pressed) and change its width with every language change.
After some time of using automatic detection starts to go insane.
I hope it will be improved soon, that I would use it completely.