I’ve had Activity Stream enabled for quite some time on my browser (Beta channel). Just today I realized that when I opened a new tab, then watched the two Firefox processes, they combined to use around 50-75% CPU while doing nothing. If I loaded a webpage then let the browser sit, CPU use drops to near zero.
I disabled Activity Stream and this behaviour wasn’t present any more. I re-enabled Activity Stream, and the problem seems to have gone away.
Definitely not of “bug report” caliber, but maybe a point of interest for someone. Maybe it’s happening to other people and they’re not realizing it.
Running OS X 10.11.6 and Firefox 55.0b1