It is a functionally beautiful add-on. It does all I expected it to do. However, I noticed a big hit on Firefox performance when it is enabled and there are a lot of tabs open (which is why I needed it in the first place).
In summary, I had to disable it and switched to "Tab sidebar", which is not as feature packed, but keeps my Firefox quick.

Great extensions, works perfectly...
...but i just noticed that your extension have low activity always on background, even when sidebar is not even open. Firefox own task manager show "low (0.4)" all the time. I am aware that all extensions are sometimes active on background, but not all the time.

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.

i was hoping it would show a summary of all tabs in all windows, not just the current window, please?
thank you.

Amazing plugin. Combining my 2 favourite plugins; 'tree style tabs' with 'Multi-account containers'! If you love and use both of these plugins, you will love this plugin!

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!

Works as expected.