I have been using Waterfox Classic as my default browser on half a dozen boxes since 2017, on Ubuntu, openSUSE, Windows 10 and macOS. AMA. :-)
I customised Firefox fairly heavily, with an assortment of over a dozen addons I'd just accumulated over the years. Firefox Quantum broke all but 2 of them and destroyed the program's usefulness to me.
Waterfox imported my profile intact and allows me to keep my customisations.
I have a vertical tab bar on the left (flat _not_ hierarchical) which is also merged with the bookmarks toolbar, also flattened. I have two download managers, one for ordinary small downloads in a tab, and another that can resume downloads, segment and parallelise large downloads, detect and bypass redirect pages and so on. Another addon automatically detects [Next] links and preloads the content into the first page, so you a big single page instead of multiple sub-pages. Another supplies anonymous login details from a shared public database. Another sorts and dedupes my several thousand bookmarks.
Oh, and on Ubuntu with Unity, Waterfox's menu bar automatically appears in my top panel. Firefox for some baffling reason hides its menu bar, but if enabled, it doesn't go into the top panel and instead wastes a strip of my laptop's fairly small screen.
And so on and so on. Firefox Quantum destroyed all this and delivers an IMHO fairly poor user experience which is as bad as Chrome's and in no way preferable to Chrome.
I have found some workarounds, for instance a vertical tab bar, but I have to enable a setting that enables user customisation, then I have to add a subdirectory to my profile and put some CSS in it in order to hide the crappy horizontal tabs and the pointless header row in the sidebar. It's a pain. WebExtensions are sad crippled little things compared to the power of XUL and they don't allow you to to really change the UI of the browser.
My impression is that today's Firefox developers don't really know or understand what people did with the tools that turn-of-the-century Mozilla provided us with. Since they didn't have a clue what was possible, they decided to rip the whole lot out and fob us off with pathetic Chrome-style extensions, Chrome-style tabs, and Chrome-style lack of customisability.
So I switched to a power user's browser that still lets me do this stuff.
There is no cost to me and contrary to what other commenters claim I can see no performance difference whatsoever. I have kept Firefox around on my Linux boxes because Github bitches about Waterfox, and apart from being much more clunky, Firefox 93 gives me nothing at all. No better speed, no better stability, about 1% of the selection of addons and they're crippled things.
TBH I thought this would be a temporary thing, but it's been four years now and Waterfox Classic is still going strong.
Mozilla, OTOH, has totally lost its way and seems to be flailing.