Thanks! Works great, what I need!

does as it says

Does exactly what it is supposed to. Really simple and easy to use. I use this instead of keeping a ton of tabs/windows open and it is a great way to save RAM.

Takes a little figuring out, but does exactly what I want it to.

Smart, small & beautiful add-on. Clever replacement of 'tab groups' for FF57.

Tried a short list of half a dozen selected potential candidates : none satisfied me. Was about to quit searching and roll back to FF56 - to stick to my previous replacement (not compatible with FF57) of 'Tab groups', when it came to my mind that THE solution should equal/manage each tab group with/in its own dedicated window. So I tried a last search with the keyword "window" instead of “tabs”. And 'Window Saver' saved my day.
Thanks bit-tinker !

Give it a try, and consider changing your habits by following the above mentioned ‘rule’ : ONE group of tabs = ONE dedicated window. This is not an issue with the improved performances of FF57. Moreover, this allows easy and more robust management of your tabs (saved as bookmarks !) : by relying on built-in features of FF, and complementary add-ons...

TIps :
1. Ctrl-Shift-N
Reopen the last closed window. Or the previous ones by hitting these keys more than once.
Useful for when you close a window and realize that you should have overridden the saved tabs from a previous session, or save them under a new name.
2. Ctrl-B
Open the bookmarks sidebar.
Useful to manage your saved “window sessions”/"tab groups" without loading them. And for renaming them : the only way actually. Addition, deletion and reordering are synchronized both directions with the add-on drop-down list.
3. Multiple Tab Handler
https://addons.mozilla.org/en-US/firefox/addon/multiple-tab-handler/
Nice complement to view the whole list of tabs when too many of them : tabs become too short to read their titles. Besides : offers to perform actions on multiple tabs.
4. "Lazy loading” of tabs - for when restoring saved sessions with many of them...
I tried two “load on select” add-ons. Unfortunately, both are not yet usable for now - independently of “Window Saver” :
• They show bookmarks URL rather than their name in the tabs title.
• Tabs are loaded in reverse order of bookmarks.
So, wait & see ...

Suggestions for additional features :
1. When opening a saved session, highlight its name in the drop down list. Better : prepend it to the name of the active page/tab in the window title. Both additions would be even better ;-)
2. Renaming of a saved session from within the drop-down list.
3. When opening the bookmarks sidebar from a window where a saved session has been restored, unfold the tree down to the folder where it was saved. An option that could be enabled/disabled...
4. Option for just ignoring “restricted pages”, rather than systematically loading a placeholder...

There are several extensions to save session. But save session into Firefox bookmarks really good idea. It add flexability to work with stored sessions, because Firefox functionality can be used when the extention UI is not enough, such as edit stored session or add stored session to current window. In my opinion, the last option it is worth adding to the extension menu.