After nothing helped me with enabling the spellchecks in one-line text input fields, I played around and found this solution that worked.

1. Type about:config in the address field.
2. Filter "spell" after you get the access.
3. 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

4. Close the tab and refresh the page to activate the spellcheck on it.