So apparently Firefox is getting another UI redesign, which based on early mockups follows the whole "rounded corners, pills, separate blocks with distinct borders" look that everyone is going for now. Personally I'm not a big fan of this design style but I'll get used to it just like every redesign. I suppose they feel they need to do this to keep pace with the fashion of the time and to look consistent with what most users' operating systems will look like, lest people perceive it as dated.

Copied one screenshot for alt-text, which is reportedly an early mockup that may not represent the final design, etc etc.

https://www.neowin.net/news/mozilla-is-working-on-a-big-firefox-redesign-here-is-what-it-looks-like/