Try running a memory test to make sure you don't have problems with your physical RAM; in Windows Vista there is a built-in memory test which should suit your needs, but for XP/other you might need to use a memtest86 or memtest64 boot disc. This may not be your problem, but for me Firefox was always the "Canary in a Coal Mine" that detected when I was about to have memory problems.
I have also noticed that some newer computers tend to have components (motherboards, specifically) that are very sensitive to dust, and a can of compressed air might do wonders; about every 2-3 months I open up my case, thoroughly dust everything including the DIMMs themselves and the ports into which they fit. Sometimes, in fact, this will seem to "correct" memory problems as it eliminates electrical noise. DO NOT ATTEMPT THIS IF YOU ARE NOT SURE WHAT YOU ARE DOING; IF YOU DO NOT KNOW HOW TO GROUND YOURSELF PROPERLY YOU CAN AND WILL KILL YOUR RAM.