Definitely performance improvements.
What's important is what was not changed.
The developers kept the old image handling behavior, that is, you can turn off downloading images, then selectively show any individual image by right clicking. This is handy when shopping at Lands' End and TigerDirect.
The new versions ALWAYS download images (squid tells me so). Your choices are to always view them, or never be able to see any of them, but regardless of your choice, the browser downloads them anyway. This really increases network traffic and slows loading, even with pipelining.
As for cookies, I prefer the old behavior of being prompted for each cookie.
Third, the menus were broken up. Cookies, java, javascript, CSS, and images were on one page. Now these security choices are scattered all over the menu structure.
These design choices were made for the convenience of the advertisers. If choices are framed this way, they figure you'll just relent and turn everything on, rather than hassle with turning things on and off.
We could have stayed with IE if we wanted hassles. Version 4.8 has its place.
UserAgent: Mozilla/4.8 [en] (Win98; U)