This is a must-have extension! Tabs should be vertical by default because it makes having many tabs more readable in a single-window.

One small request: retain the zoom level and tree expansion when re-opening Container Tabs or closing the Sidebar.

This extension could save some space by:
(1) limiting text-wrapping to a certain number of lines,
(2) expanding the area that says "drag a tab here to pin it" only when dragging,
(3) having only collapsed groups show the category titlebar/icon while making the colored strip next to any tab collapse its group when clicked, and
(4) facilitating a new-tab and close-tab with separate action buttons that work even when Sidebar is closed.

I just wish this add-on knew about container hiding.

Its a good start. I like the extension but not so much to give 5 stars just yet. But still great work. To suggestions Mordynak wrote I'd add few below:
1) Customizing the containers as you'd like. At the moment there are only 5 categories. I'd like to modify or remove some them completely and replaced with my own.
2) Possibility to hide tabs in containers from the browser's tab bar. Possibility to show only these which are inside of focused container.

Cheers

Its a nice idea but it lacks some polish.

Suggestions
It would be nice if we could choose the theme for it. Or atleast just have it use the selected Firefox theme. Currently its a dark theme?? It should match the History and Bookmarks sidebar theme.

It would also be nice to be able to adjust the vertical padding between tabs. Compact, Normal and Touch maybe? It could perhaps use your Firefox settings for this.
Have a look at the History or Bookmarks sidebars for example. They practically show the same ammount of information you need on this sidebar, except they take up far less vertical space.

Lastly. An option for pinned tabs not to appear at the top of the list, but instead at the top of the container in which they are apart of. Perhaps with an icon or someway of discerning between them and regular tabs.

Overall I really like the Addon. Its the best I've found that shows groups of tabs like it does. I just think it could do with a little more work.

Keep up the great work!

just installed Container Tabs Sidebar. will come back if rating changes. i always wanted a way to see what containers are in use. thanks for the effort, Maciej Mionskowski.

Nice idea, but not working on my setup (OSX 10.12.6 (16G1314) FF 60.0.1 )

error msg in console: Content Security Policy: The page’s settings blocked the loading of a resource at self (“style-src”). Dunno if it is related.

EDIT: great, works now!