It was good that it was removed, as it was useless to put it mildly
You're describing the "live bookmarks" feature, which was only one part of the RSS functionality that was removed from Firefox. I never used live bookmarks, but I used the feed discovery and subscription functionality all the time -- only some of which can now be restored with addons.
In the past it was even possible to subscribe with whatever addon you prefered from the firefox RSS preview directly, but that was already lost after the jump to WebExtensions.
So what's your point?
[1] - https://addons.mozilla.org/en-US/firefox/addon/awesome-rss/
[2] - https://newsboat.org/
I use Thunderbird as well, and that has RSS support but to me that seems counter to my workflow. Typically I'll be on some site and want to subscribe, and then view in my browser. Doesn't really make sense to be in an email client.
On my phone I use Flym[2]
[0] https://addons.mozilla.org/en-US/firefox/collections/4757633...
1) There also doesn't seem to be a way to sort them by date or alphabetical order or domain.[0][1]
2) It seems like it should be possible to 'group' feeds, but I couldn't figure out how.
3) When Firefox updates certain things break
I think as a result of this I will try out newsboat. I found this tutorial[2] that seems to explain it nicely.
[0]: https://github.com/brief-rss/brief/issues/208
Another great new extension is Want My RSS because it's compatible with 10 RSS readers for direct subscribtion (you can add your own when you have a self-hosted reader for example). Feed detection and feed display is also great! https://addons.mozilla.org/en-US/firefox/addon/want-my-rss/