I can already see in the Resource Monitor that the CPU Maximum Frequency (blue line in CPU graph) is not at 100%.
Windows underclocks (basically reduces your CPU clock speed) when it is idling or not doing anything that is CPU intensive.
The PC then needs to supply the CPU with a smaller voltage,
and the CPU usage is a percentage of the maximum clock frequency.
Yours seems to be at about 80%, so your CPU speed is already reduced to 80% of its capacity.
I can also see high peaks in disk I/O.
Correct me if I'm wrong, but I would guess that you're using a portable computer while not connected to electricity. Windows then reduces performance in order to save on power, so anything you do is slowed down to start with. Windows 7 is especially designed to conserve power and prolong battery life.
I would suggest to connect the computer to the power mains, and/or change the power option to High performance. See if this article helps:
How to Manage Windows 7 Power Settings
You also need to accept that video-encoding is resource-intensive, as is Firefox.