At the time, Chrome was actually a lot faster than Firefox at loading the Gmail web interface. Firefox was also faster than Firefox ESR. I've noticed on lower end hardware that newer is often better with browsers, which is not what I would have expected.

Pale Moon and K-Meleon are really just using an old Gecko engine fork, aren't they? I think Waterfox is the same.

Otter Browser is using QtWebEngine, which is Blink, so it's Chromium with a different UI.

At this point, as far as I can tell, if it successfully renders the modern web, it's a Firefox or Chrome fork anyway. None of the lightweight browsers ever seem to improve my life any.

Compare RAM usage of Pale Moon or K-Meleon with Chrome. This is important for old PCs with 2-4GB RAM. Also Otter Browser is based on QtWebKit-ng.