This problem occured on the latest firefox update on windows 10. I noticed that my CPU usage was high up when I had two tabs open with YouTube and a news site. I have an intel core i7 with eight threads and firefox used consistently 25% percent as reported by windows task manager. To find the problem I started firefox in safe mode, to make sure that it had nothing to do with my add-ons. In safe mode firefox still casued my laptop to overheat. I searched the internet for a solution and someone mentioned the accesibility services. Indeed, firefox has by default disabled the option "access to accesibility services is forbidden". So I toggled this option and since my problem has been solved, I wonder what would've caused this? What was firefox doing when this option was disabled? Perhaps more users are affected by this issue. I hope that I've given enough detail. I tried to look at about:memory, but I couldn't pinpoint the problem there.