I've issues with firefox on linux, a lot of them, it gets dark and unresponsive sometimes without any reason and then just get back 15 sec later to normal state. On one of the sites where there is a lot of text content when I close the tab whole browser gets unresponsive for seconds, on youtube most of the time video get looped in couple of frames while sound is playing normally. I use chrome on linux also, no problems there..
The unresponsiveness might be memory related, though it's impossible to know for sure. Have you tried altering the 'swappiness' sysctl setting? The default it 60 out of 100, where higher means earlier swapping to disk. Another culprit might be too high dirty page thresholds. There's also the drastic step of deleting your firefox profile folder and starting over...
probably memory swapping. on one of my inherited PCs, I recently realized I had been running with only 2 GB for over a year! I get the same black out from the OS (Ubuntu) with massive amounts of swaps seen from vmstat 1.
