If Firefox is running, do not exit Firefox. Please back up your session history files to a safe location for potential recovery/salvage operations. Details in a moment.

If Firefox is not running, do not start Firefox. There's a different way to back up your session files.

If Firefox is running: open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Return

In the first table on the page, click the "Show in Finder" button.

In the window that launches, scroll down and double-click into the sessionstore-backups folder. Save all files here to a safe location, such as your Desktop. If not too much time has passed, we may be able to use them to recover your lost tabs.

If Firefox is not running: use the steps in this article to open your profile folder: Profiles - Where Firefox stores your bookmarks, passwords and other user data.

Then scroll down and double-click into the sessionstore-backups folder. Save all files here to a safe location, such as your Desktop. If not too much time has passed, we may be able to use them to recover your lost tabs.

The kinds of files you may find among your sessionstore files are:

• recovery.js: the windows and tabs in your currently live Firefox session (or, if Firefox crashed, your last session)
• recovery.bak: a backup copy of recovery.js
• previous.js: the windows and tabs in your last Firefox session
• upgrade.js-build_id: the windows and tabs in the Firefox session that was live at the time of your last update (earlier this week, most likely)

Could you take a look at what you have and the date/time of the various files to see whether you think any of them would have the missing tabs?

Also, do you have an Old Firefox Data folder on your desktop? If so, check inside for any recent session history files.

Firefox 44 got released very recently, so you may have an upgrade.js-build_id file in the sessionstore-backups folder that got created with that update.

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

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

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

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/