Firefox was getting killed by the OOM killer for me at random intervals ranging from days to weeks of uptime. I upgraded to version 6 and it seemed to get worse. I ended up moving my ~/.mozilla directory aside, starting fresh and the memory gobbling stopped. Then I migrated the custom settings I had made under ~/.mozilla that I still understood, and it has remained stable ever since. I cannot even guess how many years and incarnations of mozilla browsers had been using that, or what could possibly have been causing the problem, but I am back to my normal usage; 12-20 tabs open running continuously.