Sometimes my Mac Snow Leopard (version 10.6.1) randomly freezes up, and I can move my mouse, but I have the spinning beach ball of death. I can also move windows, but nothing is responding. This happens for several seconds, and then everything returns to normal. I use Firefox, which I know is a memory hog, so is that causing it? How can I fix this?

Just a note, this was happening before I upgraded to Snow Leopard. I got my computer in late June 2009, Unibody Macbook Pro 13".

I encounter the same situation as you too sometimes, but for me it's because of my external hard disk. It goes to sleep if there's no activity on it, but when there's a need for it, I get the beach ball too while waiting for the hard disk to spin up again, which usually take around 3-5 seconds, or maybe a little bit more.

I had same problem before, and it was RAM(Memory module) caused that problem.
I replaced new RAM and it has been fixed.

Anyway, I think you should bring your Mac to genius bar(Apple retail store) if possible...

Hi , it is my glad to help you .
1- Please delete some program , that you don't want it.
2- Delete cookies files .
3- Increace your comuter memory (RAM)

Thank you