W3hobbyist

Those red dotted lines below words can really irritate and break your concentration while typing. So if you wish to disable it, here is how you should:

1. Type about:config in the address bar. A list of settings and their values appears.
2. Scroll down to layout.spellcheckDefault setting and double click it. A prompt box appears. This box is pre-filled with its existing value which is ‘1′ in this case.
3. Change that ‘1′ to ‘0′ and click OK.
4. Restart firefox(optional)

If you ever need that spell check again but only occasionally, you can turn it on by right-clicking in the form field and checking ‘Spell check this field’ option.