I fixed my spelling problem in Firefox today:

The symptom: a bajillion variants of English and Spanish dictionaries are installed/available, and Firefox would always select Spannish (Cuba) on a restart -- then highlight mispelllings for me, in Spanish.

The problem: My default spelling language is set to en_AU for English (Australia), but this doesn't seem to be honoured on a restart. I have to select it from the spelling checker Languages list.

The fix: I'm unsure why, but changing the Firefox setting spellchecker.dictionary_path (in about:config) from /usr/share/myspell, which is present on my openSUSE 13.2, to /usr/share/aspell, which is not (I was thinking — incorrectly — that I was setting the name of an external spell checker program…) resolved this. Maybe Firefox was picking up system-wide settings in myspell? Other programs seem to spell correctly though… so it's a mystery to me.