Updated: February 9, 2019
Back in the good ole days, Firefox had a wealth of excellent, powerful extensions. Among them, Tab Mix Plus with a superb built-in session manager. Come Firefox Quantum (57 onwards) and WebExtensions, a lot of goodies have gone away, forever. We are left with diminished functionality.
One of the things that I've been hunting after the most is a flexible session manager akin to the old stuff, with the ability to manage multiple sessions in a smart, simple, elegant way. I think I've finally found an addon that does the trick. It's called Session Sync, and I'm happy enough to actually write a whole article about this.
I tested half a dozen other extensions before finally focusing on this one. The rest simply didn't do the job well. There's an alpha quality version of a new Tab Mix Plus, too, but it's more of a stub, a shell, just a starter for what might one day become a great tool for tab and session management.
Once the extension is installed, it will have its own little icon next to the address bar and the search box. Click to expand. The interface looks a little busy, but it's not too difficult to master. Let's begin with the right pane. Here, you will have the list of all open tabs. Click the Heart button, and it will save the session. The list of sessions will show in the left pane. You can save multiple windows (private tabs will be excluded).
You now have the option to restore your session if you like - in the same window next to the open tabs, or in a new window. You can also merge tabs from/with existing sessions, as well as merge entire sessions. The addon also supports Firefox account sync, which is nice - you can also manually backup your profile.
The addon is configurable, but most of the settings are cosmetic. You can style the look how you want it. Then, you can enable lazy loading restore (the tabs won't actually be loaded until you activate them). There's also an option to configure automatic session save plus frequency, and configure hotkeys. All in all, quite neat and practical, and does the job. The one thing missing is an import/export function - other than the account sync. It would be nice to be able to manually save the settings and the sessions. After all, this will probably be some text file whatnot, like JSON or whatever. Even better, the ability to import OLD session from TMP or similar extensions would be a great thing. Today, if you want the old stuff, you need to dig through ugly text files and parse URL from awful-looking structs. But it's doable, which means it can be scripted. Hence, the import.
One small step for WebExtensions, one giant step for humanity. It is actually such a sad state of affairs that we're slowly piecing together a fraction of the glory, power and flexibility that was the old Firefox, with so much good stuff still missing. Sorely missing. The beauty of the classic Firefox was that no matter how wild your imagination was, there was an extension for that. Not anymore.
Session management is such an important functionality. Even more than bookmarks, I think. And yet, there's nothing really of that kind, other than brute saving all your currently open tabs as a bookmarks folder. Perhaps one day, we will have come full circle. At the moment, Session Sync is a breath of fresh air in a room full state of rehashed technologies. Take care, fellow Firefoxians.