Are you running the Safari 3.0 beta on your Mac? Almost all of the problems on sites like Yahoo are fixed in the latest beta version.
You might also consider trying Camino. It's by the Mozilla Corp (the company that makes Firefox), but it's designed specifically for Mac. It's a great browser that's every bit as fast as Safari.
I have to tell you that I'm not terribly impressed with Safari. Almost every web application I've ever worked with has some issue with Safari when it comes to plugins.
If you find Firefox running a bit slow on your computer, then try Camino (as has been recommended). It runs much faster because it's a direct port of the Mozilla web rendering engine (what Firefox uses) for OS X. The only difference is that you won't be able to run Firefox extensions, but for the speed difference I prefer Camino.