Right now is the best period of time yet for Firefox-based browser, especially when most alternative browsers are Chrome-based.
While there are a bunch of forks like Librewolf and Palemoon, they provide features mainly for power users like hardened privacy and tweaked user-prefs. A year ago the only fork I knew of, based on recent stable versions of Firefox and added productivity features on top was Floorp. I was very surprised at the hype and sudden popularity of Zen Browser in the past few months and have been curious why it grew so much faster than Floorp which has been around for much longer, look at the Github star graph: https://star-history.com/#zen-browser%2Fdesktop=&Date=. Zen Browser currently has 19.3K stars while Floorp has 6.1K.
Reasons I can think of are the following: heavy promotion of the browser by the devs and community on places like Reddit along with emphasizing its 'zen' philosophy, really fast development (it now has way more features than Floorp), and the Zen mods store, where you can install CSS mods.
What are your thoughts and reasons for Zen Browser becoming so popular so fast? (while its not mainstream, it did grow fast in among Firefox and power users)
Zen Browers gives a new look and feel to Firefox, while Floorp uses the same design and makes their added features look like some extenstions rather then build in features.
And I think a big advantage for Zen browser is, that it presents itself as an OSS alternative for the popular Arc browser, without mandatory account.