Check if the spell check is enabled. Go to Tools >> Options >> Advanced >> General check if Check my spelling as I type is checked.

This is checked and works on like google for example but not on hotmail for some reason ?

Firefox doesn't have "autocorrect", what Diego mentioned is SpellCheck.

ministe,, Are you sure the autocorrect on Outllook.com isn't a feature provided by that website? And it just quit working? I don't know, I haven't used Hotmail since like 1999 and never used Outlook.com.

I do know that many webmail portals have their own spellcheck feature, and have seen at least one that offered a word processor type autocorrect feature. And I recall having to disable the Firefox built-in spellcheck feature back around 2005 to use the one in the webmail portal at that time, because they didn't "play well together". As I recall, all words had red underlines, but all were actually spelled correctly.

When I looked at this problem in January it was a disaster. I'm surprised Microsoft hasn't made improvements by now. Earlier thread: Spell checker quirky running Outlook mail with Firefox.

If you use a rich text editor with Hotmail then Hotmail might be disabling the built-in Firefox spell checker.

You can also check the right-click context menu in the text area to see if "Check Spelling" is present and has a checkmark.