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!