IIRC https://addons.mozilla.org/en-US/firefox/addon/tab-groups/?s... is a replacement

Container tabs is totally different, that's about basically having multiple profiles in one window. E.g you can have the work container logged into your work email and work stuff, and your personal container logged in to your personal email and personal stuff, and they don't know about each other. Like incognito, but persistent and within the same window.

According to its developer it is the only of his extensions that may have a shot at surviving webextensions, though requires a rewrite and dropping of some of the useful features.

  Tab Groups has a shot. I took on this project after it was
  decided to remove the built-in Tab Groups from Firefox, as
  I thought it could be a good and fun learning experience;
  it hasn't been, if anything it's been stressful and
  time-consuming. I don't really use groups outside of my
  development profile, with my browsing habits I only find
  them useful to a point, they're helpful for my
  development/coding workflow, but I've used them maybe twice
  in my main profile during normal browsing.
  
  Its core functionality and basic workflow probably can be
  made into a WebExtension, but only after an almost complete
  rewrite of the code (with some major work done on Firefox's
  side as well!), and still stripped down of at least some of
  its features. Many of the new groups features I've wanted
  to add since the beginning are impossible though, for the
  same reason as I mentioned above: they either don't fit the
  scope of what can be allowed through WebExtensions or their
  implementation would be far too complex to do on my own.

They are persistent, but I don't think they are tied to a window (I mainly just use one). But your groups are kept as you left them after restarting.

I have 7 groups right now on my home machine, and that many or more on my work machine. Each has a few to a large number of tabs in it. I have about 28 in my current one. Each group contains tabs about a different topic, such as daily visit sites, searches and articles for ongoing development in a particular language, research into specific projects, or random lookups.

I also use the Tree View Tabs extension which show tabs in a hierarchical list on the left, instead of across the top. This is a better use of space for me and shows the relationships of tabs.

I am going to miss both of these severely if the XUL plugins go away this fall as they are saying. My web workflow is much more efficient with them, at least in Firefox. I like using Chrome, but with a lot of pages open I just have a squished up mass of tabs across the top that can't be easily read, it's a big bother.

Anyway, I wish there were more extensions for more browsers that improved the state of managing large groups of tabs by topic.

Last time I tried the groups that were opened when quitting firefox would reopen at next launch but you could not "reopen" a closed group. Having more than one window was asking for trouble as in risking losing all your groups and tabs because there another firefox window opened in the background or the downthemall manager window when you closed the main firefox windows.

Workaround is to always quit firefox using the ctrl+q shortcut, though at times groups will reopen with the correct number of tabs but they're all empty.

Best bet is to manually save your session at times.