See this article by Liam Proven at the Register.

Mozilla, please stop aping Chrome. Copying is rarely the way to win big. The Australis Chrome-like theme in Firefox 29 annoyed users and was a driving force behind Pale Moon. Firefox Quantum killed XUL addons, and drove The Reg FOSS desk to Waterfox Classic. Others went to Basilisk instead, while XP users have MyPal.

Even Microsoft’s Chrome-based Edge has vertical tabs. Firefox doesn’t, and to make them well, you must mess around with config files, although we still feel it’s worth it. The Vivaldi browser shows that there is still plenty of room to outdo Chrome on features, and a market of people who want for them.

Can we please have a Firefox that leans into being the browser for power users? Bundle some of the more powerful extensions that survived your self-inflicted extension extinction event. Vertical or tree-structured tabs, on any edge. Menu bars and hotkeys. Multithreaded downloads, even integrated BitTorrent support. Perhaps experiment with this stuff in the Firefox developer edition. Bring back the customizability it once had, which you’ve been steadily removing for years.

Yes yes yes!

I still use Mozilla almost exlusively on my Mac, and I use tree style vertical tabs, which are so much better, but I had to customize Mozilla so much!