The spell checker in Firefox 2.0+ has two cool features that some people may not know about. When a word is underlined in red because firefox thinks the word is misspelled, you can hover over the word and right-click with the mouse to see a list of suggested corrections. If the word is correctly spelled, but it is not in the Firefox dictionary you can add it to the dictionary. When you right-click to see the list of suggested spelling corrections, the last option is always "Add to dictionary". If you select this option, firefox will now know the word.