Of the many tab managers I tested, Tab Session Manager is the one I love and use most. I has all the functionality I need implemented in a lean, efficent and no-frills way. Easy to understand, trivial to learn and has never failed me (using it about a year now).

It comes with built in manual sync to your own google drive account (no 3-rd party servers or sign-ups in between Firefox and your G-drive account).

It auto-backups all your Firefox windows & tabs automatically locally (time interval can be set in user settings) and is the best browser crash-insurance to date.

Tab Session Manager is a well designed, cross-browser and efficient FLOSS extension: Free (as in free beer), Libre (as in liberte, egalite, fraternite) and Open Source Software, with code access on github. It is well documented and in continuous feature development and bug maintenance (due to the ever evolving browser API changes).

My warm gratitude to the main developer @sienori on github.com