See::

• https://support.mozilla.org/en-US/kb/restore-previous-session

In case you use "Clear history when Firefox closes".

• do not clear the "Browsing history"

• Settings -> Privacy & Security
  Firefox will: "Use custom settings for history":
  [X] "Clear history when Firefox closes" -> Settings
• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

• using "Clear history when Firefox closes" in Firefox 102+ honors exceptions and keeps cookies with an allow exception, previous versions removed all cookies
• clearing "Site settings" clears exceptions for cookies, images, pop-up windows, and software installation and exceptions for passwords and other website specific data

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox Title bar, especially if you have multiple windows or Private Browsing windows open to prevent losing tabs in unnoticed windows.

• "3-bar" menu button -> Exit (Power button)
• Windows: File -> Exit
• Mac: Firefox -> Quit Firefox
• Linux: File -> Quit

I have a similar problem: if I close Firefox from the Windows 11 taskbar (right-click icon, close all windows), when I start it next, the window I was in returns with all its tabs restored, but the other windows are not there and I have to go re-open them from Firefox Menu-> History->Recently Closed Windows .

Works correctly if I quit using the Firefox menu or Ctrl-Shift-Q. Still, the issue is weird.