Firefox 2.0 has a built in spell checker. Write your post as you normally would and it highlights misspelled words with a red underline.
Unfortunately, it does nothing for grammar. For those that have Microsoft Office's Word, you can type your posts in Word, have Word spell and grammar check it, copy it and paste it here.
Your - possessive, ex: Your car.
You're - contraction for you are. ex: You're great.
Ur - chat room speak for either your or you're. ex: Ur butt looks great on web cam.