I think this may be related to the new Firefox, but I'm not sure. It's a relatively new phenomenon... a couple of weeks now, i think.

Whenever I perform a browser task, like open a window or download a file or (sometimes)try to play a video in Firefox, the action will start but it only runs for a moment or two and then hangs. I have found that the action will begin again as soon as I move my mouse or click a mouse button. So sometimes I will have to move my mouse in circles waiting for the requested action to finish.