It simply provides a smoother, faster experience than other browsers (on the websites that I use, at least), especially so on older platforms.
I didn't like FF before the update. My all-time favorite browser is Opera 12, but it doesn't work (or grinds to a halt) on many websites now, and its closest relatives (Chropera and Vivaldi) don't offer that much on Chrome. I tried Edge, too, but both Edge and Chrome are too resource-hungry for the 2008 machine I have at home. "Too many open tabs" wasn't a big problem in the dial-up days, and I feel like it shouldn't be today, with supposedly much better tech.
So far I don't have much to complain about the new FF; obviously, the extensions are not there yet, but I don't rely on these anyway. All in all, I switched to FF for technical reasons more so than moral ones (each year, it feels more and more dirty to be enveloped in the "unevil" ecosystem, but that's a separate issue).
Hope this addresses the questions that some might have before trying it for themselves.