I have definitely noticed my laptop fans spinning up whenever I do Youtube on Firefox on Windows. I just figured the GPU acceleration was broken, but this makes sense. Certainly not the first time Windows Defender has consumed extraordinary amounts of system resources for simple tasks.
I've noticed that AWS Console will spin up the fans on my MBP running Firefox, specifically on the EC2 screen. None of the other Console screens spin up the fans like that. Viewing about:performance always shows the AWS tab running full tilt to the point I've jokingly assumed they're trying to spin up an instance via WASM ;-)