In both the screenshots you posted your CPU was 15-20% loaded (15% user, 5% system). That's much more than "idle", an "idle" Mac should have total CPU load of just a few %
A CPU load 15% will produce CPU temperatures of 60-70 C
It looks like Firefox is the culprit, perhaps you are viewing sites with lots of Flash or animated GIFs?
If you close all programs especially FF and leave your Mac for a few minutes, what CPU load does Activity Monitor show, and what does your temperature change to?
(your total CPU load should drop to just a few percent and your temperature should drop significantly)