Does it still happen in Troubleshoot Mode?

You can check for problems with the sessionstore.jsonlz4 file in the Firefox profile folder that stores session data.

• rename/remove sessionstore.jsonlz4

You may also have to rename the sessionstore-backups folder.

When sessionstore.jsonlz4 is deleted you lose pinned-tabs and open tabs and you will have to recreate them, so if necessary make a note or bookmark them. sessionstore.jsonlz4 is only present in the Firefox profile folder when Firefox is closed.

You can use the button on the "Help -> More Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page (Root directory).

• Help -> More Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

I am having the same problem and it is very annoying. Many Times when I want to close the extra browser by mistake I close my regular browser and lose the tabs. The solution posted is very complicated. I hope Firefox will come up with a simple solution.

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

You can check "History -> Restore Previous Session" and "History -> Recently Closed Windows/Tabs".

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

Thanks. I followed you instructions " Windows: File -> Exit " but now I have three browsers open.