Browser detection is becoming an outdated (and unreliable) practice more and more these days. Instead, you should be looking at Object Detection for the functionality that you require.
Basically, if your visitors' User Agent supports the bookmarking object (addFavorite) then show the button, otherwise display a 'manual' message instead. The bookmarking object is currently supported in IE, Mozilla/Firefox, and Opera.
If you Google for "addFavorite" then you should get enough results and guidance to be able to write the script...
- Mike
Edit: This page might be especially useful -
http://labnol.blogspot.com/2006/01/a...k-firefox.html