Well, the tabbed browsing in Mozilla (or Opera for that matter) alone is enough to get me to stop using IE (I often find myself mistakenly middle-clicking on links in IE. argh.).
As far as speed, Mozilla has been benchmarked as "slower" than IE several times.
FYI, it has been discovered that IE "cheats" TCP/IP packets when communicating with IIS servers (drops half of the handshake and just starts sucking data), which can make it seem faster on some sites.
Speed alone however, shouldn't influence your decision, imho. Use whatever browser has the functionality you like. For example, I prefer IE's handling of "View source" in context menus as opposed to Mozilla's implementation (frames are EVIL). But I prefer Mozilla's adherence to web standards (read: W3C) over IE's lame support, especially when it comes to CSS.