Sometimes you need to turn the Spell Checker off for awhile. For example, when writing in another language you haven't installed the dictionary for it, or entering programming code. Sometimes the spell checker is a real nuisance.
If you turn the spell checker off with NoSpell and you then need to spell check something, the context (right-click) will allow you to spell check a single text box.
NOTE: If you use NoSpell to turn the spell checker off or on it will take effect on any new pages, or you will have to reload the page you are on.
NoSpell is not compatible with Electrolysis, so perhaps it is time to retire it.
Go to about:preferences#advanced
Go to the General Tab and uncheck/check "Check my spelling as I type"
You won't need this add-on, but it saves you a couple of clicks if you do it a lot.