I'm overwhelmingly a user of Palemoon rather than Firefox. I am extremely satisfied with Palemoon, particularly given the stewardship Mozilla has provided of late.
I hate what Firefox has become. At this point, It's a marketing company with a technology product, not a technology company. I don't like third party applications being inflicted on me. I don't like the state of flux in the UI that has existed since Firefox 26, the change or removal of features I've been using for years. I don't like arbitrary, zero-notice changes to features I'm using. These are all bad things.
But I'm going to stick with a Mozilla-derived browser for as long as humanly possible because all the alternatives seem worse. I like leaving tabs open. Browsers that use One Process-per-tab will annihilate my available RAM. Chrome (-ium), Opera and Safari all lack privacy and security-related addons that I won't surf without. Edge, with no addon support at all and forthcoming "We're gonna try to use Chrome's!", is a complete non-starter. I need Java in a browser for IT operations tasks. Anecdotally, I see as many issues with fake/bad addons in Chrome's Extensions as I did with BHOs in IE6's heyday.
Chrome has gone from the simple, lightweight option to a bloated mess that duplicates a lot of OS functions. I don't even want to load on a low-spec machines any more. I know it's the web's new favorite, but I'd rather take the ham-fisted marketing driven Mozilla mismanagement any day than live in an ecosystem where Noscript and RequestPolicy aren't really available.