Marilyn:
What do you mean by "I am not giving up an entire browser to snipe, but would add an extension."?
You can use AuctionSniper in most browsers, including IE, Mozilla, Firefox, Opera, Safari... and I know for sure that the "Snipe It Now" link works in IE and Mozilla/Firefox. So you ceratinly don't need to give up a browser to use AS.
That said, a snipe extension for Firefox is a kinda cool idea.