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).

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
• C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile>\

See also:

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

It's too late i closed it but recently my firefox crashed just a few days ago, can i get back my tabs from the crashed?

I solved it by manually edit sessionstore using upgrade.js.