Hi ivan,
...I isn't just one core at 100%, but 1 core for a few seconds then it switches to another core for a few seconds then another one. It works through all 4 cores and the only way to kill it is the big red switch.
A new profile, without any addons, doesn't do anything to change the behaviour (the computer has been switched off more times in testing than it has since I built it 
normally up 24/7 since it also runs the FTP server).
Alright, so this behaviour is very similar to what I have seen on numerous occasions as we were testing the FF49.x builds. Here is what I found out to be the two contributors:
1) high-res timer
I have the following in my CONFIG.SYS, without this my Phenom II X6 cores are constantly jumping around: SET NSPR_OS2_NO_HIRES_TIMER=1. This appears to be very much dependent on your hardware. Some people say it works great letting FF haver access to that timer, others (like me) have to disable it, or else the CPU cores are constantly going crazy.
2) layout.frame_rate FF profile setting
This controls how frequently FF attempts to re-paint the screen and based on my understanding other platforms allows FF to avoid flicker as you scroll pages up and down (the back end code ties-in to things like monitor vsync refresh rates, etc.) However, for us this simply does not exist (the process making use of such an API to make the determination of what rates to try to sync up to), although an effort was started to develop this because SNAP drivers after all do in fact have that API, again, limitted to SNAP only AFAIK.
Now, if you are going to play with this setting here is what the options are (copy/paste from an on-line article):
1) '-1' use hardware vsync to tick the refresh driver
2) '0' means: Enable "ASAP mode", which ticks the refresh driver as fast as possible and disables vsync
3) Values larger than zero mean: Use a software timer to tick the refresh driver at the specified rate (N times per second)
On my machine I have it set to '0', which is the default value in FF 45.9.0. The tradeoff to using this is that as you refresh less often (values get progressively higher) that CPU load diminishes however FF starts to feel un-responsive...so it is literally slow to respond to your keyboard input, page up/down commands, etc.
Anyways, try these two things out. Do them one at a time, I would recommend #1 (high-res timer setup) first.
