1. Tabing , tabing , tabing specaily when comparing to M$ IE ..
2. FASt
3. Pop-up Blocker
4. extended RSS support
5. enhanced security
6. improved IE compatibility.
7. new Find toolbar

Till now nothing .But when i used it some times under M$ it get Rarely extensions problems..but this is nothing Comparing to common and repeated IE problems .

when i'm at work and stucking on M$..there nothing better than use Fire Fox for surfing the net .

Mozilla Firefox is full-featured browser for Windows that makes browsing more efficient than ever before ( copiedbut that the True :-P )

if u for any reason using M$ products , At Least try to use FOSS versions under windows like all Mozilla Suite,Open office,X-chat,Fire Fox,Thunder Bird,Gaim.. ;-)
Long Live FreeDom