This extension has proved very useful. A lot of comments are saying it doesn't work as expected, so here's a guide to how I'm using it:
1. To get the folder view shown in the screenshots there are three ways: 1) press Alt-Shift-T, 2) open the Popup Menu, select the three-dot menu and then click "Tab Manager". Both of those options open the Tab Manager in a new tab. Or: 3) in the Popup Menu, click Saved Tabs at the top, this opens the Tab Manager/folder view in the popup menu itself.
2. Be aware that no tabs will appear in the Tab Manager/folder view until you Save some.
3. In the popup menu's Open Tab view you can manipulate all open tabs, such as moving a selection of tabs to a new window (one of the things I particularly wanted), closing a bunch of tabs at once, pinning them, and of course saving them.
4. Whenever you Save a tab, it gets saved to the Tab Manager (Folder view). When saving you can enter a folder name to organize them in a hierarchy, and at any time you can drag-and-drop both tabs and folders to re-organize.
5. Other comments mention that the Save Tab shortcut (Alt-T) doesn't work. For me, on Mac OS, it does save the tab and doesn't open the Tools menu (I guess it's a different shortcut on Mac OS). However pressing Alt-T to save the tab also closes that tab. I don't know if this intentional or not, but I find it annoying - I don't necessarily want to close every tab I save. So I find it better to save tabs using the Popup menu.
6. In the Popup Menu, be aware that tabs are organised by window, and the window names are set according to whichever tab is active. It would be nicer if there was something like "Window #1", #2 etc against them, but you get used to it.
And that's it really. It provides the ability to easily manage tabs in bulk (close, pin, etc), to move tabs between windows and to new windows, to save a bunch of tabs and then open them again later, and provides easy searching through all your open or saved tabs. If you need those features, it's a really useful addon. It also allows for import/export, but I've not tried that personally.
I'm giving four, not five stars because the UI could be a little better, and it's a shame that Alt-T to save tabs also closes them at the same time. It would also be nice, as others have mentioned, if keyboard shortcuts could be re-configured.
But it's a very useful addon which I wish I had installed a while ago. I often open a bunch of tabs for a particular purpose, and just do so in whichever window is open. Then later I regret having so many different tabs, for different purposes, in a single window. This addon makes it very easy to move a bunch of tabs to a different window, and/or to save them for later access.