Running Windows 8 x64 (but problem also occured when I had Windows 7 x64) and latest version of firefox. All plug-ins up to date.
My problem is a bit different than others, since we use 2 profiles on my computer. My wife's profile, firefox is always using 40-50% cpu usage. When I switch over to my profile, firefox doesnt use anything out of the ordinary.
For example, right now I am in my profile using firefox to create this ticket. Task manager shows firefox in my wife's profile using 45% CPU. When I switch over, she just has facebook open.
Its the exact same firefox version & plug-ins, so its not a difference in code. My profile is administrator and she is standard user.
This has been a constant issue when running Windows 7 x64 last year, to a clean install of Windows 8 x64 for a month or two now. She obviously closes firefox or opens different websites, but without a doubt, her profile and firefox is always using almost half the CPU and memory of 800k+