55 here because 56 breaks Mozilla's own Tab Center add-on. I see no mention of new APIs for add-ons in the release notes...
Individual tab hiding is already implemented in Firefox 59 (Beta / Developer Edition), but currently behind the "extensions.webextensions.tabhide.enabled" pref as we iron out final UX / security / privacy concerns. Still, tangible progress is happening.
It's just too bad that it's afterwards. I don't see why it was necessary to toss an old system before the new one is finished. It's not as if they can't coexist.
Sorry if this is a little harsh. The hard work is appreciated and as a developer myself, I know about technical debt and stuff. This is a huge project with loads of people to please. I just hope in the future we can build the new house before tearing down the old.
Mozilla dropped support for Tab Center a while before Quantum IIRC. Another addon took its place, but then was dropped by its developer with Quantum. Supposedly [1] will allow transition from the older addons. I'm looking forward to finding out once 59 is out.
[1]https://addons.mozilla.org/en-US/firefox/addon/tab-groups/ [2]https://bugzilla.mozilla.org/show_bug.cgi?id=1423725
Edit: Looking at the status:
> Status: NEW → RESOLVED 4 days ago
That could have made it into 58. Or they can still add it: why wait? They started this quick major-release cycle to release features sooner, but I don't see why they can't just release them when they're, y'know, ready.