"at the same time that Firefox is sliding into irrelevancy it's becoming a better browser. It's faster than it's ever been and uses less memory – less than its replacement, Chrome."
Citation required methinks. The best way to test a browser's efficiency is to make it run on restricted hardware. On my venerable 2008 Macbook (with RAM and SSD upgrade), my preferred Firefox hees and haws whilst Chrome is snappy and responsive. Sad to say there is no contest.
On my main machines I run a combination of Vivaldi and Firefox. I should note that Vivaldi isn't actually a whole lot better. The actual Vivaldi process is slim, but then it spawns lots of helpers. In any case, you'd be hard pressed to back up the assertion that Firefox is appreciably more efficient than Chrome or Chromium-based browsers. And they badly lagged behind on multi-threading and other technologies.
Like many others I avoid Chrome for it's all-in-one bar, lack of decent menus and general unfriendly UI, and have become frustrated with Mozilla's efforts to turn Firefox into a Gecko-based Chrome-a-like.
For the privacy minded, Brave is worth a look (I've only used it on iOS, not got around to taking the desktop version for a spin). Chromium based with ad-blockers built in as standard.