I have heavily used and to some extent still do use Palemoon. They started off as a Firefox fork that reversed some of Mozilla's more egregiously stupid decisions and back-to-back-to-back releases that broke everything. They then got lost when scripting changes and Web Assembly became a thing - they were seriously overwhelmed. Instead of getting to work they got angry. They couldn't admit they were in over their heads, and any time a user went on their forum talking about more and more web sites that didn't work, the authors, moderators, and even senior community members would gank them. It got bad - really bad. They were blaming web sites for using WASM, blaming users for wanting to use those sites, and angry at the world that their pet browser was bit-rotting before their eyes. Blaming everyone except themselves.
I have to admit, I was pleasantly surprised when a year or two ago this seemed to somewhat change. They seemed to decide to get busy instead of just getting angry, and they started pulling Palemoon up by its bootstraps. Or were trying to. It is, unfortunately, still not usable without going to extreme measures, like using a stream-editing plugin with secret-sauce search and replace patterns to tweak web sites on-the fly replacing problematic constructs. You can find forum threads where users will pass back and forth arcane search-and-replace scripts that make advanced regex look readable, and sometimes it can be made to get some web sites working (for some definitions of 'working'). Even then, Github, All Discourse-based forums, and many other places like banking sites are buggy, or just outright unusable. Their efforts seem to have stalled and Palemoon is losing ground again.
They also still shoot themselves in the foot with weirdness too, like they block all tor access from every domain they use. And not just the forum - If you use tor, you can't even upgrade the browser, or view release notes. I was going to try and get Palemoon some money from the Tor project to possibly get adopted as the official Tor browser, something given Palemoon's stated privacy goals seemed like a good match. But they will have none of it. There was some sort of knife fight between project admins some years ago and it got ugly where apparently tor was used for part of it and now their project owner is allergic to it.
All in all it was a laudable browser with laudable (stated) goals for a while. But it's still unusable, and I couldn't any longer in good conscience recommend to anyone to actually donate them money.