Seems you need the feature as much as I do, but it is not in Firefox. By default Firefox does not check spelling on single line text boxes either.

What do you type that automatically gets corrected, because I've never seen it happen in Firefox. Are you perhaps using an external editor. Is this at a particular website.

I can't find an add-on for that at addons.mozilla.org either.

Do you mean spell checking or filling in form data?

• Tools > Options > Privacy > History: "Remember search and form history"
• https://support.mozilla.com/kb/Form+autocomplete

• https://support.mozilla.com/kb/Using+the+spell+checker

Whenever I type "label:important label:inbox" in Gmail, it gets autocorrected to "label:important label:mbox".