Very usable, and looks very good. Tabs grouping works perfectly. The developer actively maintains the github repo and gives prompt feedback on the opened issues and suggestions.

Similar to edge's vertical tabs, good job

To make current tabs stand out in Dark Theme, can set the border to white under SideBar Tabs > Options > Sidebar Theme > Active Tab > Border, set to White

Some feedback:

Option to name the groups, similar to edge's vertical tabs?

Also allow us to remove the top tab bar? Right now, I have to manually edit the userChrome.css by following this guide ( https://medium.com/@Aenon/firefox-hide-native-tabs-and-titlebar-f0b00bdbb88b )

It's pretty cool, but it lacks some features that I need.
1) for example I have like 300 cards open at the moment, it would be nice to have a button somewhere to automatically group them by domain (because I have to click on every different domain and group it separately, right now. It takes a lot of time.

2) an option to group new tabs after opening them? After I open new youtube tab (while being outside of any group), it should be automatically grouped into the "youtube" domain group. So I don't have to do that manually.

3) a button to collapse ALL groups at once?

4) another thing that is bugged - after I close the Tabs sidebar (the whole plugin sidebar) and open it again, only a few groups are kept (seems to be random ones), or only a few cards are kept within a group. Most of them need to be created again, which is troublesome. Both for normal and domain groups. Most of them disappear after closing the sidebar. Only a few are kept.

thank you for your work

Pretty good but has some usability issues. Primarily, the list scrolls to the bottom whenever I make a new tab. Also it's a shame you can't double-click to open a new tab, or middle-click to close a tab (and remove the close button at the end). It doesn't show whether a tab is loaded or not. It works well with Containers, showing the icon at the end - though it would be really cool if you could filter the list by container (i.e. an option to show one container at a time).

Reply: I installed the pre-release (2.1.0a1) but there's no difference re double-click or middle-click. Nor does that about:config change do anything re showing if a tab is unloaded - it only shows if it is loading. Fair enough re containers.

Exactly what I needed! Can you maybe do it so it auto groups by domain? Example wikipedia.org 20 pages open and it automatically groups them?

"Customize Theme" does not work. Every next choice switches the previous one to default. Need fix.

EDIT
That was fast! Now it works. Thanks.

this is the best looking and most responsive feeling vertical tab addon in my opinion, thank you!

Simple and gets the job done, the closest you will find to the old IE styled tab groups.

current version used:
https://github.com/asamuzaK/sidebarTabs/tree/master/dest

Obviously it's a bit early to rate, but on the Mac, one cannot "ctrl+click" as this is the same as right click. Expected alternative cmd+click and option+click do not seem to work.