The Spellbound [Development] extension provides live spell checking as you type for any web forms or text boxes in Firefox. Similar to Microsoft Word, Spellbound will underline misspelled words providing users with one-click spell check options. Click on the underlined word and a drop down menu of possible replacements is displayed. If you are certain the word is correct, add it to the Spellbound library. You can find more information as well as a download link on the Spellbound thread at the Mozillazine forums.