I have it again with MozillaFirefox-44.0-12.2.x86_64: Firefox eats 130 to 160% CPU on a 4-core machine, making the fans scream. This happened after I had visited https://bugs.documentfoundation.org, but maybe that's just a conincidence: I was not using Firefox at all; it was just sitting there with a bunch of tabs being open. What I'd like to have is an "about:cpu".