Empty your cache. If you're running 10.6.4, I'm assuming you have Safari 5.0, but if not upgrade. I find that it's a dozen times faster than 4.0. Also, check your Activity Monitor. You might have some runaway apps that are sucking up CPU power. If so (and you don't need them) quit or force quit them.
SnowLeopard Cache Cleaner is also great for giving your Macbook a nice speed boost.
And of course, you can always upgrade your RAM. But nobody likes spending more money when you've already paid $1000 for your computer.
Also, in my opinion, Firefox is the worst browser I've ever used on OS X. It sucks my CPU down, hogs all the memory, and runs slow. I use Chrome as an alternate browser if need be.