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.

Video logs by Lena Nechet - videos from daily life.