In a "restore session" window I accidently clicked on "start new session" rather than "restore previous session". Is there a way to restore/recover tabs from previous session after starting new session?

You can check the [http://kb.mozillazine.org/Profile_folder_-_Firefox Firefox Profile Folder] to see if there is a file sessionstore.bak If you find such a file and it has the date/time from around that crash then make a backup copy elsewhere and rename the file to [http://kb.mozillazine.org/sessionstore.js sessionstore.js] See http://kb.mozillazine.org/Session_Restore