IE 8 is faster than IE 7 in all cases, except where it counts: in opening new tabs (effectively starting a new IE process, and all attached ActiveX modules with it). This is a known design deficience that Google worked around in Chrome by using Webkit (arguably the lightest and fastest HTML/CSS parsing and rendering engine out there), and that Firefox and IE 7 don't encounter because all tabs are kept inside the same process.
However, I personally stay with Firefox for several reasons: it's actually pretty hard to go and crash it on purpose, and when that DOES happen, Firefox's session restore has until now never failed me (right down to lengthy texts typed in a textbox being restored to the last character) - so, tab isolation isn't that useful for me.
Next, although IE 8 comes with vastly improved developer tools (from a meaningless Jscript error message box to a Firebug clone), Firefox still comes with better developer tools.
Then, Firefox doesn't focus on this or that trendy piece of code's acceleration (IE 8 focused on rendering the 20 most used websites fast), but on all of them.
Last, and that's probably what counts most for me, Firefox runs on all my machines natively: be it Windows 2000 to Seven, or Mac OS X or GNU/Linux, I can take my Firefox profile and port it from one machine to the next. On the other hand, taking an IE 8 'profile' and porting it to a Windows 2000 machine doesn't work (no IE 8 on Windows 2000), moving it from an XP machine to a Vista one is bound to create bugs, and moving off-Windows is a no-go.