PC won't sleep when firefox is open

I've noticed since I've switched to chrome that my pc won't go to sleep at all when any type of firefox tab is open. I've run the command prompt to confirm that it is only firefox which is preventing my pc from sleeping and I also haven't downloaded anything new since switching over. I've seen that sometimes media tabs cause this however even if only the google home page is open it still prevents sleep. This is incredibly frustrating as before I could leave my pc whenever and it would auto sleep and now sometimes it'll run all night. What can I do to fix this. As far as I know there is nothing in the firefox settings page to fix this.