When you close a normal window and Firefox is still running with other windows open (either PB mode or other windows like the Library) then that normal window falls under undo closed windows (History -> Recently Closed Tabs/Windows) and this isn't reopened automatically on a next start.

Did you check "History -> Restore Previous Session" for that lost session?

Griffel said

Hello. The problem:

I'm have one Firefox window in normal mode, with several tabs opened. Settings is set that Firefox should start the previous session when launched.

I open one more window in private mode. Then I try to close the main window, and receive a request, if I want to close the opened tabs. I tried to agree, and when I launched Firefox again, there was no previously opened tabs. How should I fix this issue?

As I know, the window in private mode shouldn't affect the session in main window in any way.

I haven't tried it the way you're doing, but Control-SHIFT-Q or File > Exit or in the Main Menu (3-Horizontal Lines at upper-right) 'Exit' will close the Session and the amount of browser windows and Tabs is remembered, if you have it set this way.

Clicking on the 'X' at the upper-right corner closes that browser window - so it's NOT a remembered Session for that browser window.

I guess you have to open FF with one or more browser windows and then open a Private Window. (Or if one regular and one Private Window were opened, 'Exiting' and restarting will be a remembered Session?)

~Pj