⚠ This addon extends and works best with Tree Style Tab. (There is also support for other similar addons such as Sidebery.) This extension can bookmark and restore tree structures for Tree Style Tab. Use the context menu item on a parent tab to bookmark its tree. You can also select multiple tabs with the Multiple Tab Handler extension and bookmark the tree information for all of them via that extensions context menu. In Firefox 64 and later you can use Firefox own multi-select feature. To restore trees from bookmarks you right click on a bookmark folder saved with this extension and click the "Restore Tree" item. Due to API limitations this context menu item is only visible for bookmarks in the toolbar and in the bookmark toolbar button menu. The context menu item can't be used for bookmarks in the sidebar or in the library window. This limitation has been fixed in Firefox 66 and later where the context menu item does work on bookmarks in the sidebar and in the library window. If you don't want to restore the the contents of a whole bookmark folder then you can use the context menu item on a bookmark instead. This extension will then look at the folder that the bookmark is in and figure out what bookmarks it is parent for and open them as child tabs for the bookmark's tab. If the bookmark's tab wouldn't have any children it will instead look at what bookmark represents its parent and open that together with its child tabs. The bookmark folder that you use this in needs to have been saved with this extension. Note: this extension can't restore trees that were bookmarked with the legacy version of Tree Style Tab. Bookmarks support in Tree Style Tab v3.2.0: Tree Style Tab v3.2.0 and later have support for saving tree structures to bookmarks without installing this extension. The way Tree Style Tab saves tree data is different from how this extension used to save it. Tree Style Tab prefixes bookmark titles with a number of ">" characters while this extension used bookmark separators to save the tree structure. Version 4.0 of this extension adds support for restoring and saving tree data using Tree Style Tab's data format but by default it will still use bookmark separators so that it is easier to restore any bookmarks that are already saved using that strategy. There is also an extra context menu item for bookmarks that can migrate data from one bookmark format to another, note that this context menu item is disabled by default and needs to be enabled in the extension's option page. Comparison with Tree Style Tab v3.4.8:

This extension can be used to bookmark partial tree, that is to say a parent tab and a selection of its child tabs. If you select multiple tabs and not all of a parent tab's children are selected then those tabs won't be bookmarked while Tree Style Tab will bookmark the entire trees even if not all of the tabs are selected.
Tree Style Tab's restore tree functionality can only be used for entire bookmark folders while this extension can be used to also restore a single tree inside a bookmark folder. (Edit: Tree Style Tab's developer has released a separate addon TST Open Bookmarks as Partial Tree that offers this functionality.)
The unloaded tabs restored by this extension uses the bookmarks' titles as the names of the created tabs until they are loaded while Tree Style Tab uses the URLs.
Tree Style Tab offers a menu to choose where you want to bookmark your tabs while this extension always saves them to the default bookmark location.
Tree Style Tab has better compatibility with containers.