Not running more than one antivirus program, correct? Antivirus programs do not play nice with each other.
When your computer is idle, is it on any particular page(s). Note if you happen to be on a page that automatically fires either flash video or a lot of flash ads. Both will send your CPU through the roof. Poor performance can happen on a such a page whether on a Mac or PC.
Firefox has been notorious for a while imo for producing a browser that is a bit of a pig. I use pretty much every browser for my work and have found Firefox to consistently be the poorest performing, even when compared with IE 10 and 11.
Do you have a bunch of seemingly little utility programs running on your machine in your task bar? There may be stuff you have had since you have had your machine that you really do not need to be running at startup.
And of course as others have said, check your processes and look for one eating up your CPU or memory (or both).