If you want to find out yourself, I can highly suggest using the Firefox Profiler on https://profiler.firefox.com/ - amazing tool!

Besides the devtools profiler, which may not capture everything there's a whole-application profiler for firefox, that might provide some insight: https://profiler.firefox.com/ But try the devtools one first, it's easier to use.

the f does Firefox ... same URL when opened in safari

It would be interesting to compare the safari budget/manpower. Even given that, I would imagine safari only has to support one-ish OS, and probably has help from the OS group.

In my experience (which includes using both Firefox and Chrome on a Linux VDI which is particularly CPU-constrained), I've noticed that Firefox's event loop handling seems pretty poor, and seems to redraw/paint even a blank page (and has a full core of CPU usage for doing nothing), whereas Chrome is fully-idle and has no CPU usage in the same scenario. Minimising Firefox is the only way I've found to stop it updating the VDI's content and using CPU.

Things (on the same constrained resources VDI) get even worse with actual "complicated" content: viewing the same page which contains an animated GIF, Firefox seems to prioritise playing back the content, even at the expense of noticing/obeying user-interface interaction like closing tabs and opening menus: i.e. gifs will keep playing (very slowly), but it won't notice UI clicks on its interface for many seconds seconds.

In the same scenario, Chrome responds to UI events much more quickly (very nearly instantly).

It's possible this is due to hardware acceleration or something, but as both are running in the same VDI that barely seems to have OpenGL 2.0 support (I can't run modern OpenGL apps), that seems unlikely to me...

In addition to what others have suggested you might want to check how many things you have registered under about:serviceworkers (type that in as the URL), there might be something in there running far more than is reasonable.

I’ve gotten in the habit of killing Firefox before going to bed, the processes seem to run away and peg the CPU periodically throughout the night. It seems to be generally Facebook, but other JS heavy sites have similar issues.

Chrome has the same issue. The thing is, Safari is deeply connected to OS X and its internals to save as much energy as possible. Apple can do this and tie the code to the specific hardware, kernel, and what else matters that Safari runs on as they control everything. Chrome and Firefox cannot.

I would not be surprised if someone tells me that parts of Safari hook into private APIs in the kernel, the graphics driver and the other parts of the graphic and input stack to achieve the (measurable!) performance advantage over other browsers. For me personally, this is uncompetitive behavior that should be punished, but oh well, anti-trust legislation isn't exactly something that got much use over the last decades :(