Back when I tried sidebery, there was some weird issue where either shift-click or right clicking didn't work on mac, and that turned me off. I just tried it again, and both work fine now.
One other feature that is nice for me is the ability to collapse the sidebar to just the tab icons. It's a nice middle ground between being able to see what I have open and getting a full screen experience.
TST and Sidebery are both fantastic extensions, I don't think they do anything wrong. For whatever reason though, the FF native implementation worked for me where they didn't
The biggest benefit I've seen is that it automatically hides the old tab bar at the top. Before that, you had to dig into some hidden profile directory and modify some userchrome CSS file and modify the CSS directly hoping it would work.
I use this method personally and it works great on GNOME and KDE. First set `toolkit.legacyUserProfileCustomizations.stylesheets` to true in `about:config` then find your profile directory in `about:profiles`.
cd $FIREFOX_PROFILE_DIR
cd chrome
git clone https://github.com/MrOtherGuy/firefox-csshacks
touch userChrome.css
To me, what they shipped seemed lacking in features to both, with no real improvements.