After many years of using Firefox and other browsers, Tab Stash is the first extension that's made it possible for me to keep the number of tabs I have open under control. Its UI is thoughtfully designed to make common actions take as few clicks as possible, in contrast to using the browser's built-in tools. This extension is as indispensable as uBlock Origin. Thanks to the author, Josh!
If you've come from Google Chrome to here and are trying to find a grouping extension that's similar, this one is! Instead of grouping them in the top bar, you'll have a little sidebar. BUT, the important thing is that it will let you close out of tabs on your main slider without actually deleting them. So, if you don't like having a bunch of windows open or tabs up on your slider, you won't have to!! You can even open one tab at a time or the entire group. An absolute lifesaver for me :)
Firefox itself has a built-in feature that automatically backs up all your bookmarks (including Tab Stash's saved tabs), so having such a feature in Tab Stash itself would be redundant. You can find this in Firefox's bookmark manager, where there are options for taking a manual backup and restoring from a manual or automatic backup.
Also, Tab Stash has a deleted-items feature that will keep track of deleted folders/tabs so that if you delete something by mistake, it's easy to recover.