Whenever I try to play a Video on a website, I am getting "beach-balls" constantly, the videos don't play fluently, stopping almost every second, Firefox (and/or the OS) seems to freeze for a moment, the fan starts to make noise (which btw, also happens a lot lately when I watch any movie), and sometimes I get a message asking me whether I want or not to stop or continue executing a script. It happens mostly when using Firefox, but not only. I have both Firefox and Flash Player updated to the last version, 28 and 18.104.22.168. I am using a MacBook Pro from early 2011 on Lion. Any ideas on how I can fix this problem?