I've really enjoyed the "It's All Text" extension that allows you to use whatever editor you like to edit text fields.
https://addons.mozilla.org/en-US/firefox/addon/its-all-text/
https://addons.mozilla.org/en-US/firefox/addon/its-all-text/
A note for anyone using Firefox with Electrolysis (a.k.a e10s or content processes) enabled, e.g. Nightly users: It's All Text is partially busted in that case. E.g. see https://bugzilla.mozilla.org/show_bug.cgi?id=1042965#c2. It's still usable, though.