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
/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.