I was a true believer in IE for a long time (and I still think it's the best bet if you're trying to build an app in a corporate setting) but when Firefox came out, I changed my tune. Once you experience the Fox and all of the wonderful extensions that provide functionality you never considered in a browser, you'll never use anything else (well, unless you're forced to by...say...your companies Intranet system). It's that good.
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Wired News: The Firefox Hacks You Must Have