Hmmmmm, hard to tell, do you see high cpu usage when streaming from anyplace else? If so or if not, it's still a good idea to try Process Explorer to try and find which process or software is causing that, when you're in FireFox check under FireFox and you'll see the plugin container and the Flash Player is under that and see if it's Flash that's hogging your CPU or something else.
In your computer specs, you didn't mention what graphics card you have and what graphics card you have could definitely make a difference depending on how beefy the gpu is and if it supports video acceleration or if not then more work will be done in software by the driver than in the hardware so the cpu may have to work harder if video rendering can't be offloaded to the gpu. Have you tried updating the driver for your video card? That could help if rendering is being done by the video driver rather than in hardware by the gpu.
I'm sorry to say that I'm more of an Intel guy than an AMD guy as I've been disappointed by AMD systems I've built compared to Intel systems. So I'm not a good judge of AMD hardware and I can't tell if what you've got is good or not, I normally would not recommend a cpu that's under 3.2Ghz but it's hard to tell with AMD. Also when I build computers for home or office use I usually build them with 8GB of ram and Windows 7 will run on 4GB of ram but how well depending on what all you're running at any one time is another thing. Process Explorer will also show you graphs for CPU usage, Physical Memory, I/O and GPU usage which may or may not be helpful in trying to locate where the problem is.
My computer specs:
CPU: Intel i7-3820 3.6Ghz - quad core with hyper-threading
Motherboard: Asus Sabertooth x79 Socket LGA 2011
Ram: 16GB
Hard drives: Western Digital Black Edition 7200 RPM 2TB X 2
Windows 8.1 with update 1
Graphics Card: Asus Geforce GTX 570
Corsair Closed loop liquid CPU cooler