nettester I've been a Palemoon user for many years and even it being pretty much just a somewhat dated Firefox will simply break left and right these days (besides being awully slow due to having to emulate a lot of stuff - also doesn't help much anymore if you know all the common ducttape...) and the developers know they (like pretty much any alternative browser project) don't have to resources to fix the problems or keep up with the ever evolving web standards in general.
This is perfectly consistent with what I've been observing while using Seamonkey (formerly, my browser of choice). Until 2017 or so it was perfectly feasible to use Seamonkey and other Gecko-based browser (Palemoon, GNU IceCat, K-Meleon) as a Firefox replacement, for practically any everyday task. Extensions were almost always cross-compatible. WebkitGtk browsers were also very usable (Midori, Epiphany, vimb, Uzbl). And there was also QtWebkit, which served as base for the spectacular Qupzilla (another browser I really liked, and used, especially as I was running KDE4 and Lumina on PC-BSD and TrueOS respectively). Other common browsers like QBrowser and Rekonq were based on QtWebkit too.
Then something happened all of a sudden. Firefox released FF57 with the new Quantum engine, suddenly broke all extensions for Gecko browsers. The new multi-process engine ran fast on modern machines but turned out excruciatingly slow and heavy on older CPUs. Firefox on x86 became impossible.
At the same time, the new replacement for QtWebkit, QtWebEngine (based on Google's Blinky) was released, and Safari and Opera were refactored over it. The WebKitGTK project went through very troubled times in the 2-3 years following WSA-2015-0002, as many distribution dropped it from their repos and encouraged switching to something else. Qupzilla was refactored as Falkon (now built on the QtWebEngine) and was incorporated in KDE5 Plasma, making it unfeasible to use Falkon as a standalone alternative, without pulling the whole KDE5 distribution as dependencies.
The Web quickly evolved over web apps, become more and more cluttered and more encapsulated in walled echo chambers, and it feels like it was actively reshaped around the capabilities offered by Chromium, without taking diversity into account, and thus making the internet experience on other browsers always subpar.
At that point it became clear the everybody had to use Chrome and its derivatives, and that more tech-savy people might have used Firefox, but only because they wanted to.
