Spinning beachball and then freeze can be symptomatic of a virtual memory issue. Hence the advice above to ensure there is enough free space on the boot disk. It could also be symptomatic of a device wait (possibly killing the VM system) , and thus the advice to check the disk is OK.
But it can be symptomatic of the application going nuts and taking all the available memory. Mavericks has some aggressive new memory management techniques that may have some issues when confronted with a brain dead application.
However there are a host of other possible issues. There were a bad batch of GPUs in some Macs a few years ago - they cause crashes and Apple replaced them. There can be interactions with other third party software you might have installed. The exact model of Mac, and configuration may matter, as well as any other software you have installed. Running the activity monitor where you can see it when you try to watch a video may help isolate what is going on. Not just CPU use, but memory can be important. The fact that you have a number of CPU burning programs detected does suggest there may be a more deep problem with your machine. I tend to have the activity monitor running constantly. The Dock icon turns into a CPU use graph, which makes for very easy detection of high CPU use.
Flash is a constant problem. So the suggestion to swap to HTML5 is good.
You may want to try using Safari as well to view a bad video, to isolate Firefox as potential problem.