I can't leave Firefox open on even simple pages, without it consuming ~5% or more of the processor with one tab doing nothing. If I open numerous tabs, forget about it, Firefox will eat the processor (brand new machine, i5-7400, new Firefox install). I can leave Chrome running almost perpetually without problems with tons of tabs open. Right now I've got seven tabs open in Firefox, and it's consuming 706mb of ram, for a few stackoverflow pages and HN. I've had that resource abuse problem with Firefox essentially since the beginning, across a lot of varied PC systems.
Over the last year I've gradually stopped using Firefox because I can't stand its horrible performance any longer.
I am using the developer edition instead of stable, though, and there have been a lot of somewhat recent improvements; maybe that accounts for the difference.