[inigyou]

Doesn't Firefox still have this? It's a special kind of bookmark folder populated by RSS.

[defrost]

Moz Support says "Firefox's built-in support for web feeds and Live Bookmarks ended in December 2018 with version 64."

I checked as I use the Feedbro extension in Firefox and Chrome family browsers for holding and updating a tree of RSS feeds and thought I'd missed a built in FF feature :/