Spell check works for me on Windows/Firefox. But also, I have my Message Editor Interface set as "Enhanced Interface - Full WYSIWYG Editing"
Go to your inbox, and on the side menu find "My settings", "My Account", "General settings". Click on "General settings", and at the bottom in your "Miscellaneous Options" is where you can set your "Message Editor Interface" to WYSIWYG (What You See Is What You Get). I don't know for sure, but that might help you see spell check in your text box.