High CPU usage at rest

I have about 20 open tabs at any time, which Windows reports as 9 threads. Most are webpages I am working on. None of them have any active content, or they shouldn’t have and it’s not obvious.
Anyway, Windows reports that Firefox developer edition is using 65%-80% CPU and 15%-25% GPU at all times. I disabled all extensions, same problem. Restarted the browser, no change.
Could you please advise how I can help you?

[edit] this is a Windows 10 x64 pro (up to date) with Firefox developer edition v60.0b15 (x64), a Core i5 CPU, SSD storage and 32G RAM. GPU load is 35-40% overall, RAM usage is about 40%, CPU is at 100% all the time.