called "General: Browsing:"
https://support.mozilla.org/bm/questions/1176777
Icant find any counterpart
I dont have time to perform this other solution
Type about:config<enter> in the address bar.
If a warning screen comes up, press the Be Careful 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.