Sorry, beaten to it by Pieter (I don't know why I couldn't see his post before).

By the way, EXCELLENT script, really useful and well done.

I can tell you how to tweak it.
-Install it first.
-In FF go to Tools > Greasemonkey > Manage User Scripts
-Select "Profanity Filter"
-Click Edit
*If you've never done this before, you'll have to select a text editor. Notepad works, I really like Notepad++ http://notepad-plus.sourceforge.net/uk/site.htm
-There is a section that starts
"//sorted alpha backwords to catch bad word groupings
var badwords="...
Add whatever words you want to this section in single quotes with commas between ('word1','word2')

That is it! If you don't like the *** this script replaces bad words with, find this section: var ReplacementText = "***"; and replace the *** with whatever you want (like *expletive*).