Pros:
tabbed browsing
better w3 standards compliance than IE
puts pressure on MS and others to produce a better browser
and standards compliant websites by the non-use of IE
better security
pop up ad blocker
interesting skins if you like eye candy
better bugfix cycle/update release cycle
no MSN crap in favorites or toolbar
capable of restoring entire session if browser crashes (no lost webpages or re-typing them)
Free but worth money (unlike MS-Free which is merely worth the price you paid)
some websites designed to recognize your name and browser version
are not capable of determining browser type when you use firefox which
tells me firefox does not dish out your personal information to websites out the back door without your knowledge. IE is capable of giving a properly designed website much more information about you and your computer than your IE version and today's date...
Cons:
some sites still only work with IE
is not the de facto standard (like IE) but likely will be inside 2 years
