Can you tell us exactly or approximately if nothing, when did it start to happen? Since you are an old member here, you would say it earlier if it was from start. Also, not anyone else complaining on the same issue, maybe it can be the addons you have installed in your browser, or some other browser option you checked.
If you can provide us the addons you got installed or something like that so that we can look into problem more, that would be nice. Also, try firefox in safe mode and see if it happens again.