the only way you will see a performance difference to anything related to the internet is to get a faster internet connection, browsers dont use much resources at all (firefox is a bit of a memory hog but cpu use is still small) most browsers can run just fine on slow comps, I have seen plenty of old P2s and P3s run firefox without a problem. To see the performance difference between two CPUs nowadays you have to do more than surf the net. Try encoding video or playing games.
Current CPUs are so fast now that mediocre things are as fast as they are gonna get. Old computers used to take a few seconds to open folders or even the start menu, its instant now. If you got a faster CPU hoping to browse the internet faster then the only thing I can think of a CPU as helping is online flash games which use more cpu power than normal browsing.