- on awaking the system/ Firefox, a significant lag/ hang of around 30-40 seconds is experienced
 - this is accompanied by the «beachball», and occasionally, the blue beachball.
 - it seems as though excessive RAM is being consumed, but on checking Activity Monitor, all seems correct.
 - it gets to a stage where the lag/ hang is so bad that other apps refuse to coöperate

I'm running Mountain Lion on a 2009 Intel MacBook Pro. Since upgrading to Firefox 18.0.1 a few days ago, my system, at times, becomes unusable: - when active, Firefox is clean, supple and very fluid. - when left idle (background) or minimised to the dock for a short length of time, it becomes unresponsive. - this then has a knock-on effect on my system and renders it unusable: - on awaking the system/ Firefox, a significant lag/ hang of around 30-40 seconds is experienced - this is accompanied by the «beachball», and occasionally, the blue beachball. - it seems as though excessive RAM is being consumed, but on checking Activity Monitor, all seems correct. - it gets to a stage where the lag/ hang is so bad that other apps refuse to coöperate - the only work around I have found for the time being is a force quit (which shows that Firefox has become unresponsive - a little like a crash, but caused by no activity). I wonder if anyone else has the same/ a similar problem. What fixes you use. If it is a known bug. If anyone has managed to rectify the situation.