%APPDATA%\Mozilla\Firefox\Profiles

Well, you know from your own experience that it's not normal... I guess the question is: how did the clock get turned back? One possibility might be the Windows "system restore" feature. Otherwise, perhaps the more recent session history file became corrupted or got cleared somehow. Could you check whether you have any other session history files that might be more recent? Here's how: First: Do not exit Firefox, or if you closed it, don't re-open it. If Firefox is still up, check the History menu for "Restore Previous Session" and, if it's not grayed out, try that. If that is grayed out or doesn't help, let's back up a few files. Here's how: '''(1) To open your profile folder...''' ''If Firefox is still 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 In the first table on the page, click the "Show Folder" button. ''If Firefox is closed:'' Type or paste the following into the Windows Run dialog or the system search box and press Enter to launch Windows Explorer: %APPDATA%\Mozilla\Firefox\Profiles In that folder, do you see a semi-randomly-named folder? If so, click into it. If you find multiple such folders, find the one that was most recently updated. '''(2) Copy out session history files''' In your profile folder, scroll down and double-click into the '''sessionstore-backups''' folder. Save all files here to a safe location, such as your Documents folder. If not too much time has passed, we may be able to use them to recover your lost tabs. '''(3) What files did you find?''' 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 at the last shutdown and is still closed, 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 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? Note: By default, Windows hides the .js extension. To ensure that you are looking at the files I mentioned, you may want to turn off that feature. This article has the steps: http://windows.microsoft.com/en-us/windows/show-hide-file-name-extensions