Where did you buy your memory upgrade from? That memory speed is not what shipped with the MBP, it should read 667 Mhz I believe. I would run a memory test on your machine (you can search the forums for a program on how to do this) to make sure that RAM is performing correctly.
Also, in general, I find Firefox to be dreadfully slow...I only use it to access sites that don't support Safari.
Well, you did slow your Mac down some with that memory.
I find most browsers open slow the first time after a re-boot: in the 5-10 second range. Once loaded into memory though, it should only take 1-3 seconds to load your home page depending on the site and your connection speed.
And if you don't care for Safari, you might give Opera a try.
Firefox here on my Imac is one of the slowest to load browsers I have used. It does it on ANY machine in the house. Once it's opened it's fine, but it takes the longest to launch on both PPC and Intel Macs here and I have most every browser installed that is available for OSX.
The other users are right though, if the RAM is supposed to be 667 and you are running 533, you might run into problems.
Take a trip over to crucial.com and use their memory finder. It will match the correct memory modules to your specific computer. Once you find out exactly what specs you need, you can buy from them or another site like Newegg.com or elsewhere.
The reasoning behind this is not that you might be able to find slower rated speed but higher performing RAM, but rather when you put in slower rated RAM, the system must slow down its bus speed to match the RAM....and this in turn slows down the whole computer.