Most likely a problematic add-on (Google this, there are others like you), first start Fx in safe mode to see if it clears the problem. If that works, now for the good old fun of disabling your add-ons one by one to find which one (or more) is the culprit.
While using FF pages won't load unless I am moving my mouse or typing.
Some embedded videos won't play unless I am moving my mouse. Other
animated images won't move unless I am moving my mouse. What is causing
this, and how can I fix it?