I disagree, there is low-hanging fruit Firefox is leaving on the table. The main thing that comes to mind is tab unloading. They don't unload tabs automatically like chrome can.
I was pleasantly surprised at the tab unloading settings under "memory saver" in ungoogled-chromium.
I believe that Firefox does it but not as frequently as Chrome does, but don't quote me on it. However, I am using a "tab suspender" addon on firefox to control how fast the unloading happens on tabs that are not active.
I was pleasantly surprised at the tab unloading settings under "memory saver" in ungoogled-chromium.