You're kidding, right? You just
die('Switch to Firefox / Safari') if someone's using the wrong browser? Jesus, how long have you been doing this exactly - don't ever completely disallow access to content based on the User-Agent string, it's just silly.
There are other browsers out there too - I'm an Opera user myself, which thankfully has a feature that makes it very easy to identify itself as Firefox (or IE, etc.). But, man, it's a bit annoying and shortsighted to have something like that in your code.