First thing to check is that you have the current version of Adobe installed for firefox. I am using the beta version at present for firefox - not for internet explorer - and this works very well - download here if you are interested. The download is the green box top right hand corner.
To try to isolate the problem....
Open up a video, YouTube would be fine, when it is playing right click and uncheck the box that says enable hardware acceleration if it is checked and see if this helps.
You may need to disable the plug-in container - follow the steps in this article that shows you how to do it.