I don't know what happened but I suggest...

First: Do not exit Firefox and if Firefox is closed, do not start Firefox.

Please back up your session history files to a safe location for potential recovery/salvage operations.

If Firefox is running:

You can 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 Enter/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 Documents folder or 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:

Check this article for how to find your profile folder and sessionstore-backups folder: Profiles - Where Firefox stores your bookmarks, passwords and other user data. Also look for sessionstore.js at the main level of the profile folder (not inside sessionstore-backups).

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

• sessionstore.js: the windows and tabs from the immediately previous session that shut down normally
• 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 from 15 seconds or more earlier
• 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

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?

Oh wait, are you using the Session Manager extension? It may have its own separate stash of session history files. I'm not familiar with how that works.

Note that Firefox 36 isn't the current release and using that release make you vulnerable to security flaws.

You can find the full version of the current Firefox release (42.0) in all languages and all operating systems here:

• https://www.mozilla.org/en-US/firefox/all/