@delve I've had one sidebar reader or another installed since Firefox 1.0.
Harbi Xenu (Possibly? Name was some meaningless thing. Stopped working with Firefox 2.0)
Sage 2 (Sage didn't update so someone forked it)
Sage (Sage 2 didn't update, and Sage started updating again)
Drop Feeds (current, since Sage stopped updating)
the mastodon instance at cybre.space is retired
see the end-of-life plan for details: https://cybre.space/~chr/cybre-space-eol