[mynameisvlad]

Yes, it would. It's not based completely on performance gains you may see in new updates. Profiles bloat up over time with data (history, cookies, cache, etc) and the bigger it is, the more the browser has to process. So, over time, your browser will slow down. A fresh install of a new browser would result in all this data not existing. If it doesn't have to parse hundreds of megs of files, it'll generally be faster.

[Karunamon]

All of those things are easily cleared, however. If I do a clear history/cache/cookies in Firefox, there's still a lot of miscellaneous data floating around in my profile, to the point where, last I recall, the Firefox team still recommends you outright dump your profile in case of difficulties.