In search of: FireFox spell checker add-on

Because I am constantly writing I am always on the lookout for new WordPress plugins or FireFox add-ons that will make my life simpler. For the most part I am a fairly decent speller but after several hours of writing I start to experience some sort of brain block and can’t remember how to spell anything. The other day I was writing a post that included the word Phentermine and couldn’t for the life of me think how it might be spelled. I would really love a FireFox spell check add-on if anyone knows of one that works with the newest version.