Some websites have their pages set up to disable the built-it spellchecker. (They want you to use theirs, I guess) Just to be sure, right-click in an input box and make sure the checker is checked. May I suggest; Grammarly for Firefox by Grammarly https://addons.mozilla.org/en-US/firefox/addon/grammarly-1/ Grammarly will help you communicate more effectively. As you type, Grammarly flags mistakes and helps you make sure your messages, documents, and social media posts are clear, mistake-free, and impactful.