To activate the ability of firefox to spell check single line fields, do the following:
- Bring up your about:config page. You can do this by typing “about:config” in your address bar and clicking on “I’ll be careful, I promise” button
- In the â€œFilterâ€ field type layout.spellcheckDefault.
- As you can see, the integer value is 1. Double-click on it, then enter 2 as its new value.
- Click OK.