Firefox: Spell Check Single line text boxes also

Firefox comes with spell checker which checks for spelling errors in the multi line text boxes. You can make Firefox to spell check single line text boxes also like email subject lines, etc by changing config entry.

Type about:config in a new tab, then go to the key layout.spellcheckDefault. It's value is 1 by default, which means spell check multi line text boxes only.

0 - Disable Spell check.
1 - Check multi line text boxes only.
2 - Check all text boxes, including single line text boxes.

Change the key value to 2. No more spelling errors in the email subject lines.