Anecdotally it uses about 1.5% CPU on my laptop with the current Firefox stable on Windows 10 20H2. Admittedly this is a machine with an 8 Core / 16 Thread CPU, but that still feels reasonable. Firefox overall is reporting around 3.5% usage with a YouTube video playing in 1080p fullscreen, Discord and three other open (and loaded) tabs.
Exactly the same setup even. I was struggling to find the "About Firefox" option earlier, but I checked again (why is it in the help sub-menu?) and I also have 84.0.1.
