Comment 9 for bug 1026869

I think this happens when you do not have any dictionary installed as a Firefox plugin. When I was only relying on the system wide spell checker (hunspell in my case) the default language selection of the spell checker always was always wrong (notice that now you do not have dictionary tab in the add-on settings page). Since I write in Dutch, French, Danish and English, this was a real pain. When I installed one dictionary as a Firefox plugin, the default language selection worked as expected.

For the record, this is also mentioned on StackExchange (http://stackoverflow.com/questions/26936792/change-firefox-spell-check-default-language/29446115, http://stackoverflow.com/questions/21542515/change-default-language-on-firefox/29446353, http://askubuntu.com/questions/50049/how-to-use-american-english-spelling-dictionary-in-firefox), and the Arch Linux Wiki (https://wiki.archlinux.org/index.php/Firefox#Firefox_doesn.27t_remember_default_spell_check_language).