Typically right-click on the text box and look for the spell check options. Should be able to turn it onn and off there.
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I too depend on Firefox for my spilling ;-)
If you mean spellcheck, for checking the spelling what you type into the forums here, well, I know that for me, that spellcheck is task my browser, Firefox, is doing.
If you mean spellcheck, for your word processor application, e.g. LibreOffice Writer, or MS Orifice, or whatever, well you need to find out how, for that application.
Probably the best way to do this is to ask Google, like:
"turn on spellcheck in [myapplication]"
where [myapplication] stands in for Firefox, or LibreOffice, or whatever.
I mean ask Google to search for to tell you how to do it.
It won't turn it on for you. You have to do that part yourself.