You can try "Clear the startup cache..." via the "Help -> More Troubleshooting Information" (about:support) page.

• https://support.mozilla.org/en-US/kb/use-troubleshooting-information-page-fix-firefox

You can try to reset extensions.databaseSchema in about:config (i.e. click the trashcan icon on this line).

• https://support.mozilla.org/en-US/kb/about-config-editor-firefox

I have tried both approaches. No dice. The languages appear when I add them, disappear when I reload.

Just to be sure, you are installing Dictionaries and not language packs. The language packs are for changing the language of the user interface like the menus.

Yes, I have installed dictionaries. Installing them makes it appear, but it disappears when I restart.

Do you still see them installed in the extension manager (about:addons -> Dictionaries) and in "Help -> More Troubleshooting Information" (about:support)?

about:addons - the dictionaries are still installed, yes

about:support - they are also visible there, yes, and set to be 'on'

Your System Details show that you have a user.js file in the profile folder to initialize prefs each time Firefox starts. The user.js file will only be present if you or other software has created this file and is normally not present.

You can check its content with a text editor (right-click: "Open with"; do not double-click). The user.js file is read each time Firefox is started and initializes preferences to the value specified in this file, so preferences set via user.js can only be changed temporarily for the current session.

You can delete the user.js file if you didn't create this file yourself.

• https://support.mozilla.org/en-US/kb/how-to-fix-preferences-wont-save

You also appear the be using userChrome.css and userContent.css.

Well, crap. Thought that might be it - deleted the file, restarted firefox, removed and readded the discionary, checked it's working, restarted and it's gone.

.... hm. It doesn't work in ANY language now, actually. Maybe that's the problem?

What was in the user.js file? Anything related to spell checking and dictionaries?

Do you see "Check Spelling" and Languages if you right-click in a text area like on this forum?

You can enable or disable spell checking globally:

• Settings -> General: Language and Appearance: "Check your spelling as you type"
• https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker

You can check in about:config whether layout.spellcheckDefault is set to 1 (multi-line) or 2 (single-line).

I'll be honest, I didn't check much in user.js before removing it to try

I do get the red squigglies for incorrect check spelling, but it happens regardless of what I type in, so it seems to be working.

Man, fascinating further complications. I decided to , screw it, I'll just turn spell check off!

.... it.... won't stay off. Keeps turning itself back on. Not even when I reboot firefox. On its own.