I had used shareaza for over 2 years,and i love it. It is open source, and searches 4 p2p networks. There are also a lot of features that some of the well known p2p clients like kazaa, winmx,etc... I also like the fact that you can download BitTorrents with it
I have used Firefox since its first release. The thing I like best about is the extentions. Most of the extensions for Firefox would need a seperate program in IE. I have the gmail log in extension, which is very helpful when I am busy, and i dont have time to go directly to gmail to check my mail. As for sites not opening in Firefox, there is an extension that lets you open any site in a IE window.