There is no language setting in Firefox for the dictionary. You can only enable or disable the spell checking globally in Option > Advanced > General

You can check this pref on the about:config page:

• spellchecker.dictionary

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

According to the help files there is supposed to be a Check Spelling and Languages option on under the Edit menu on the command bar. Is this an old help file? Why would they remove these options?

https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker?esab=a&s=Language&r=4&as=s#w_switching-dictionaries

That is the context menu that you get when you right-click in a text area like used on this forum.

Note that websites can disable spell checking, so those entries might be missing in some cases. The Language item is only present if Check Spelling has a tick and is enabled.