This because the newest Firefox is doing it. In trying to stop crashes and slowdowns, separate instances are created.

Thank you for the reply. Is there any way to stop Firefox from launching multiple instances?

Yes, but that's above my paygrade. I called for more help.

Did you check the home page setting?

You can check the home page setting.

• Options/Preferences -> General -> Startup: Home page
• https://support.mozilla.org/kb/How+to+set+the+home+page

Firefox supports multiple home pages separated by '|' (pipe) symbols.

You can check for problems with the sessionstore.js file in the profile folder that stores session data. When sessionstore.js is deleted you lose pinned tabs and Tab Groups and open tabs and you will have to recreate them, so if necessary make a note or bookmark them.

You can use this button to go to the current Firefox profile folder:

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Firefox uses the sessionstore.js file in the profile folder to store session data, but this file is only present when Firefox is closed. The sessionstore.js file is created from recovery.js when you close Firefox and is removed and copied to previous.js when you start Firefox to make is possible to restore the session at any time.

Backup the session files in the sessionstore-backups folder in the Firefox Profile Folder to make sure not to lose possible important session data. Do NOT close Firefox when Firefox is already running.

You will normally find these files in the sessionstore-backups folder:

previous.js (cleanBackup: copy of sessionstore.js from previous session that was loaded successfully)
recovery.js (latest version of the sessionstore written during runtime)
recovery.bak (previous version of the sessionstore written during runtime)
upgrade.js-<build_id> (backup created during an upgrade of Firefox)

You can copy a file from the sessionstore-backups folder to the main profile and rename the file to sessionstore.js to replace the current file (make a backup copy of the current sessionstore.js).

See also:

• http://www.ghacks.net/2014/06/27/mozilla-launches-improved-session-restore-firefox-33/