Turn off grammar correction in a contenteditable div in FireFox

Question

I have contenteditable div with text on it. In firefox, there is some kind of grammar correction which underlines the text with red marking. How can I turn it off?

Answer 1

There is no grammar-checking native to Firefox. Given that every word is being marked as incorrect, it is possible that you have the wrong language selected.

To verify that you have the correct language selected for Firefox's dictionary: When typing in an editable field, right-click the input area. From the context menu, there is an option Languages. Ensure the expected language is selected.

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EDIT stealthyninja's answer below contains the method for disabling spell checking as the OP requested. This answer should not be taken as the solution.