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/

Location used for the main profile in "AppData\Roaming".

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

• http://kb.mozillazine.org/Profile_folder_-_Firefox

So does that mean since Firefox is currently open, the previous session is gone forever?

Although FF is still open, all that is showing is a small bar with the option to minimize or close. The expand icon is grayed out

So does that mean since Firefox is currently open, the previous session is gone forever?

Although FF is still open, all that is showing is a small bar with the option to minimize or close. The expand icon is grayed out

You could try to backup sessionstore.js and the files in the sessionstore-backups folder if they look large enough that they could contain useful data.

You can open the file in the Scratchpad ("3-bar" menu button or Tools > Web Developer) to check its content. Click the "Pretty Print" button on its toolbar to format the file for readability.

• https://developer.mozilla.org/Tools/Scratchpad

Also check "History > Recently Closed Tabs/Windows".

If none of the files contain useful session data then I'm afraid that this data is lost.

See also:

• Session Manager: https://addons.mozilla.org/firefox/addon/session-manager/

Is it possible that FF could unfreeze and then I could click on the top right menu to reopen last browsing session?

Did you check any of the existing files in the Scratchpad to see if any have this session data?

The files are not there