I don't use a Mac, but it's a known issue with the high-resolution ones. I believe it doesn't use the OS compositor, so it ends up redrawing too much.

https://bugzilla.mozilla.org/show_bug.cgi?id=1404042

https://bugzilla.mozilla.org/show_bug.cgi?id=1191965

I've used a few retina Macs to do various kinds of web dev. Firefox tends to do some things fastest (eg: re-layout when resizing the window), some things the slowest (eg: certain large, complex CSS animations). For most things, it's perceptually equivalent to Chrome or Safari. Some things that are fast/good in Chrome will be slow/bad in FF, and vice versa. The biggest shortcomings I currently face are (1) a lot of banding when layering lots of complex gradients, due to a poor job of dithering and (2) weird handling of certain colors on my P3 display due to the assumption that colors are in the full display space rather than sRGB.

A big problem of early Quantum Firefox on HiDPI Macs was its high battery usage. I think it got slightly better with later releases, but I don't have data to support that.

For people running in non-default scaled resolutions. In the default configuration it's somewhere in between Safari (best) and Chrome (worst).

I have never run my MacBook Pro in anything but the default scaling resolution. Quantum has been and continues to be a nightmare on my device. Unusably slow performance with high CPU utilization and occasionally spiking CPU utilization to 100% for prolonged periods. Usually long enough that I am forced to reboot the laptop.

Unfortunately, a non-native scaling mode is the default on new Mac laptops now. You need to switch to one of the custom scaling modes to get a true, native 2x.

I've heard this but haven't seen it on any new hardware — I thought it was limited to 2016 or so.