You might find this answer helpful: https://support.mozilla.org/en-US/que.../975459

You can check the setting of the spellchecker.dictionary pref on the about:config page and if the pref shows user set (bold) then try to reset the via the right-click context menu to the default value.

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

If you do not keep changes after a restart or otherwise have problems with preferences, see:

• http://kb.mozillazine.org/Preferences_not_saved

capitansquid said

I added a spanish dictionary to firefox as I sometimes email spanish speakers. Since then the spell checker has stuck to spanish as the default language every time I restart the browser. As soon as I select english, it stays until I close the browser. But goes back to spanish when I open a new window. I've tried every setting I could find, and I've also tried deleting and reinstalling both languages via add dictionary. I'm at a loss. Can anyone offer some advice? Thanks

This seems to have worked. THank you. Totally forgot about the config screen. Tried every menu I could find. Sometimes you gotta get under the hood for effect. Cheers.

Unfortunately, it ended up failing. As soon as I selected spanish on the next spell check it defaulted to it again and would not revert back to english no matter how often I selected it as the dictionary. I can revert it by using the same process as above, but again. It's only a temp fix. Is there no way to allow temporary use of dictionaries? This is really annoying.