MacBook Pro Less Stable Since Leopard?

Since I got the OS upgrade, my system's stability has gone to hell and my desktop freezes whenever I have multiple apps open and have lots of tabs going in Firefox. The new update didn't do anything to help things. Just this morning I had a system freeze that ended in my laptop screen fading to white, which was really weird and kinda freaky.

Anyone else having problems with Leopard? And do any Mac experts out there suspect my problems might have something to do with my system settings/what-have-you, and are unrelated to the new OS?

By the way, this past summer I added 3rd party RAM to my machine, in case that might be connected to the problem somehow. Thing is, I didn't start having system-related performance issues until I got Leopard. Would taking the laptop in to the Genius bar be a good idea?

Any advice is appreciated!

Mindflux, I upgraded over Tiger. Who knows, maybe that's a factor, too.

edit: I forgot to mention that I only have 2GB of free space right now because of music and movie files. Again, this never causes any decline in performance when I had Tiger, though.