In my experience, Ctrl-Shift-T will restore whatever was most recently closed. If it was a tab, it will restore a tab. If it was a window, it will restore the whole window.
You can use it repeatedly to reopen second to last, etc. I’m not sure how many it remembers
I responded to PastTense and somehow skipped OldGuy’s post. Sorry.
Yes, Ctl-shift-T does work to restore a window and all the tabs in Firefox.
Windows 10, FF 119.0
(for some reason I don’t have the Firefox logo on the far left, so I know nothing about it.)
Ctl-shift-n re-opens the most recently closed window.
Longer way: click on “history” (between “view” and “bookmarks” at the top of any window), then select “recently closed windows”, and click on the desired window.
The number of windows saved can be increased through “about:config” and changing the value of “browser.sessionstore.max_windows_undo”.