After nothing helped me with enabling the spellchecks in one-line text input fields, I played around and found this solution that worked.
- Type about:config in the address field.
- Filter "spell" after you get the access.
- Change these fields bu double-clicking at the value on the right to get these results:
- true and 2:
imtranslator.AutoSpell true layout.spellcheckDefault 2
- Close the tab and refresh the page to activate the spellcheck on it.