How to back up and restore firefox tree style tabs?

It turns out that my question was incorrect: "Bookmark All Tabs..." works fine for this purpose. Though the list of bookmarks under the Bookmarks menu is displayed without the tree structure, that structure is present, and the "Open All in Tabs" menu item in the Bookmarks submenu restores that tree structure.

I also use that plugin and I use the Session Manager plugin for this exact purpose. When I'm developing I often end up with a load of tabs about a particular thing and want to store them in the same tab tree order, but wouldn't want to bookmark them. I'd have tons of unwanted bookmarks.

I just save the window as a backup session and give it a relevant name, and category if required, and then when I want to retrieve them just load the session. It closes all the other tabs and opens the backup session.

I know you say you don't want to store the session, but from your description its ability to store the tab tree is just what you're looking for?