I've also used Fennec for Android, which I found indistinguishable from FF

For what it's worth, Mozilla did release a Firefox nightly test build a couple of months ago which included a proof of concept vertical tabs feature: https://www.ghacks.net/2024/04/02/mozilla-released-a-firefox-nightly-test-build-with-vertical-tabs/

Still not clear whether they're gonna implement it properly though

I use Mull on Android and so far I'm satisfied with it.

I like Waterfox. I don't care about vertical or tree style tabs, but I like the extra UI options and the reasonable defaults.

There are significantly fewer Firefox-based browsers than there are Chromium-based ones, unfortunately. Out of the ones we do have:

Floorp has much like Vivaldi gone the proprietary source-available route, so you couldn't pay me to use it.

Pale Moon is easily the most involved of the Firefox forks, being a fork of a much older version of Firefox, but I wouldn't generally recommend it for security reasons. It does have its uses, though. Waterfox Classic is in a somewhat similar boat. Security-wise Pale Moon is definitely the better of the two because it uses its own fork of Gecko which is maintained about as well as you could reasonably expect given the manpower available to the project. Waterfox Classic meanwhile has kinda just been left to rot since most development is going to regular Waterfox nowadays, so it's not maintained nearly as well as Pale Moon and it's just been collecting CVEs. But for those same reasons if all you want is the ability to use legacy XUL extensions, then Waterfox Classic is gonna have better compatibility since it hasn't been modified nearly as heavily as Pale Moon.

LibreWolf is probably the most popular Firefox fork nowadays, but it isn't much more than a Firefox equivalent to Ungoogled Chromium. Waterfox goes a little further, but not by much. Both can be good choices, but personally I haven't had much reason to switch away from upstream Firefox. LibreWolf is tempting, but I can already disable pretty much all of the Firefox BS from about:config, so I don't see the point. It's pretty much just better defaults.

My main browser is LibreWolf. I occasionally the Mullvad Browser or Tor. Mull is my favorite on mobile.

I did a test drive of Floorp as well and I ran into a similar issue. I couldn't get it working with touch input, although FF managed to smooth it out years ago. The fixes for Firefox didn't seem to work for Floorp.

It's been many, many years at this point. Which one was it that went 64-bit before Firefox proper did (Waterfox maybe)? Pretty sure I used that for a short while at the time, but memory is hazy now.

I occasionally toy with the idea of switching to SeaMonkey because I was a Mozilla Mail & News user for a long time way back when, but I switched to separate FF and Thunderbird when that was discontinued and never had the need to switch "back" to the all-in-one.