Long story short, I'm done with Pale Moon. Not only prompted by instabilities that have been going on for months now, but also censorship in their community, which I consider to be the worse offense. They really aren't any better than the others. Which makes the point of supporting their project from the ethical standpoint null and void. I've been using Pale Moon for years, even being patient with its rather laggy and 100% CPU core spike nature for the sake of some of the other conveniences and it will take some adjustment to get used to another browser, but I'll deal with it.

I'm leaning more towards something Firefox-based as I find Chromium in general chronically inflexible. While I haven't checked Vivaldi in few years now and it was impressive in its own right, AFAIK it still inherits many shortcomings, things like inability to copy profile between machines and more restrictive extension system, eg. uBO is more functional on Firefox, you can move extension buttons around, there's still some tricks you can do with userchrome scripts etc. I guess my needs are rather specific so might be why Vivaldi doesn't move me as much.

I already keep both Chromium and Firefox-based browser as a backup, that is FireDragon on Mozilla side and Thorium on Google side. Might just keep them if nothing better is found for my needs, with FireDragon as the main browser. I'd be interested if anyone knows any Chromium fork that diverges from stock in any meaningful way or at the very least lets you copy profile between machines and not lose data. I only know 3 that can do the latter with added cmd-line parameters / flags, they are Ungoogled Chromium, Thorium and Supermium.

I realize I'll never have all the bells and whistles I can have with Pale Moon, so I'll list the most important things I'm looking for first.

First, it should at least launch on my favorite computer, so no SSE 4.1 or AVX requirement enforcement. SSE3 is fine. If it throws illegal instruction on AMD Phenom II for basic operation, then it's useless. I'm fine with Windows 10, so won't ask for it to run on Windows 7 or even XP (yikes!).

ClearType font rendering, which appears to be mission impossible to find in current browsers and major reason that has kept me on Pale Moon for so long. But, as it is, I guess there's no choice other than trying to emulate it via userscript hacks adjusting CSS properties dealing with contrast. I use non-default settings on Windows with slightly bumped contrast. SeaMonkey is the only other one I'm aware of that still has normal font rendering, but sadly, their engine is way too behind to be practical. It should at very least be able to access all my sites to pay bills and the app at work, so the engine should support dynamic module imports, newer class syntax so things like class a {b;#c} don't throw errors etc., otherwise, this one would be easy to switch to.

Sane session save on quit / restore on launch with NOT loading everything, just the selected tab. Chromium is just pathetic at this, extensions can only rely on unreliable hacks to stop tabs from loading and that doesn't work at random. I recently tried one of the tab session management extensions rather than relying on native session restore + Lazy Tabs and it was worse, tabs didn't have navigation history preserved. Seriously?

Something more than barebones tab management. If nothing else, context menu with an option to unload tabs on demand and some visual indicator which tabs are active or not. Seems Firefox is much better in this regard with Tab Mix Plus.
Also, tabs below URL bar. There was one good project that tackled many visual aspects, CustomCSSforFx, but it's a lot of work to maintain and the author stepped down. FireDragon browser has some of these out of box, so as long as it is maintained, things will be better out-of-box without manual messing with CSS.

Integration with KeePass that preferably also supports entering TOTP codes. This one is easy at least, KeePassXC-Browser works OK on both Chromium and Firefox. Meant for KeePassXC specifically rather than the original KeePass, but KeePassNatMsg plugin on KeePass side fills the gap.

Preserving modification timestamp of downloaded files. No browser really does this AFAIK, but until recent releases, it was easy with Firefox with "legacy" DownThemAll! extension. But this one probably needs a rewrite to work on current Firefox... It seems there's an extension for both Firefox and Chromium to send downloads to JDownloader2, but this one is an overkill, especially since it needs not so light process constantly running in the background for seamless experience. Download with Wget for Firefox might be an acceptable compromise.

These would be the most important. Now, some nice to haves.

Colorful tabs - there isn't anything like the old ColorfulTabs extension, is it? You can have tabs randomly colored, colored by domain, manual...

Navigation sounds to have audible cue when using backward / forward functions. There's a nice legacy extension for it, reads settings from Windows registry and plays the sound you've set there. I tried one or two "modern" extensions, they had sound hardcoded (suppose could rip it open and replace it, but it's ugly) and the one on Firefox launched WebAudio context running in the background, which tends to force high resolution timers, which tipped me off a bit.

NPAPI - I don't buy the whole insecure deprecated BS. I'll take proper PDF reader in a browser tab over PDF.js any day. And you get to run Flash too, forget the fake, slow and incomplete Ruffle. Or Java. There's probably more, but yeah, the powers that be decided to kill it. Apparently NPAPI is alive in China and 360 Extreme Explorer supports it. But it's another Chromium...

Flag display indicating the country where the server is from. Seems only Firefox has saner extension for it that doesn't query online service for it all the time.

Well, that's all that comes to mind right now. Any thoughts?