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

Note: Some software, like Advanced SystemCare with Surfing Protection, can protect files in the Firefox profile folder against changes. If you have such software then check the settings or uninstall this software.

Type about:config<enter> in the address bar. If a warning screen comes up, press the I Accept the Risk button. At the top of the screen is a search bar.

Type: layout.spellcheckDefault Its values are; 0 Spell Checker Off 1 Check Only Multi-Line Input Boxes. 2 Check Multi And Single Line Input Boxes.

Also, check the spellchecker.dictionary entry.

https://support.mozilla.org/en-US/kb/how-do-i-use-firefox-spell-checker

i don't use any "helper" software which protects my system from me - those are Windows-isms, and this is Linux.

i explicitly set my spellchecker.dictionary to en_US the last time this happened to me (2-3 weeks ago), thinking that would resolve it. It's still set to that value.

layout.spellcheckDefault is set 1 (unmodified).

Some Linux distro provided builds of Firefox can come with a long list of additional Dictionaries and Language Packs packages. You could try and uninstall what you do not need in the package manager.

The en-US locale of Firefox from www.mozilla.org/firefox/all/ only comes with the en-US dictionary for example.

You can check these locations for globally installed dictionaries:

/usr/share/hunspell/
/usr/share/myspell/

Mint Linux has apparently inexplicably installed a dozen or more language packs, including Australian English, but that doesn't tell me (A) why FF would switch dictionaries on me at random times and (B) why the one FF switches to isn't in the list of dictionaries it offers me in the preferences screen (that list only includes US, UK, and CA).

Language packs (about:addons -> Languages) have nothing to do with dictionaries (about:addons -> Dictionaries).

See also "Language and Appearance":

• https://support.mozilla.org/en-US/kb/firefox-options-preferences-and-settings

According to the "Languages" tool in Mint Linux, only 3 languages are installed: en_US, en_UK and de_DE (which is okay - i live in Germany). That information conflicts with the much larger pile of dictionary files under /usr/share/hunspell/.

i can do "apt remove hunspell-en-au", but that doesn't explain (nor excuse) FF's random dictionary-swapping behaviour.

cor-el said

Language packs (about:addons -> Languages) have nothing to do with dictionaries (about:addons -> Dictionaries).

about:addons -> Dictionaries says i *only* have US English installed.

Firefox just did it again - randomly swapped the spellchecker to Australian English. It seems unlikely that it's a coincidence that Australian is the first variant of English which appears in the spellcheck language selection list.

Update: the way i noticed is because *everything* in text fields suddenly got a red squiggly line under it... because i uninstalled the en_AU dictionary yesterday, hoping it would stop this behaviour from happening.