From Lifehacker:
Firefox with Greasemonkey: Filter out the nastiness of the net with the Profanity Filter script for Greasemonkey.
By default, the script scans every page you visit against a pre-defined list of naughty words, then replaces any matches with three little asterisks. To give it a try, I created my own vulgar little sentence, which the profanity filter turned into: “This *** is the ***. Every *** on the planet should install this ***.” Pretty harmless, huh?
Sorry for all of you still using IE. I don’t know of anything that lets you do this. If anyone has a hack, feel free to correct me.
Download Firefox
Download the Firefox add-on