Sidebar Tabs от asamuzaK

Display tabs vertically in the sidebar.
Tabs can be grouped.

Browser settings

To match icon colors to your theme, enable the experimental feature as follows:
* Visit about:config.
* Search for svg.context-properties.content.enabled and set the value to true.

Tab groups

Group tabs:
* Select tabs by Shift + click or Ctrl + click.
* Drag one of the selected tabs and Shift + drop it on the tab you want to group.
* Or from the context menu (right click on one of the selected tabs), select "Tab Group" -> "Group Selected Tabs".

Add group label:
* From the context menu, select "Tab Group" -> "Show Group Label" and edit.

Collapse / expand tab group:
* Click on the colored part will toggle collapsed / expanded state.
* Or from the context menu, select "Tab Group" -> "Collapse (Expand) Tab Group".

Cancel tab group:
* From the context menu, select "Tab Group" -> "Ungroup tabs".

For further info, see Github repository.