Firefox save session?

Question

Browsers

I would like the second instance of Firefox to save all open tabs when closed.
There is Ubuntu, on which Firefox is open with a bunch of tabs, in the Firefox settings there is a checkbox "Restore previous session" and it works - I closed the browser, opened it again - all the tabs are in place.
BUT, when I open another Firefox window (which also naturally has the "Restore previous session" checkbox), then when this window is closed, all tabs are lost. How to make firefox remember tabs in all its windows?

Answer 1

make some profiles
see

$ firefox --help
...
  -P <profile>       Start with <profile>.
  --profile <path>   Start with profile at <path>.
  --ProfileManager   Start with ProfileManager.