Usually, the History menu will list closed windows you can restore below the grayed out Restore Previous Session. Anything listed?

Before closing Firefox, please back up your session history files to a safe location for potential recovery/salvage operations.

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.

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. 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 is 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

You might also find files Firefox created if it could not update recovery.js (but since Firefox 33 is so new, I don't know what they would be named).

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 window?

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

Regarding your first comment, the restore windows option opened another copy of what I already had, not the second window which is now gone.

Regarding the rest of the message, it will be some time before I will get a chance to try them, as computer time is a small part of my day.

I have closed and opened FF many times and always all windows opened automatically. Earlier in the day Windows 8 asked to do an update. That update closed both windows and reopened them when done. Later in the day when FF suggested I upgrade its own code the second window failed to reopen, a problem which I had not experienced before, therefore for which I was unprepared. Yes, I know I should always expect the unexpected!

It may be a couple days before I get a chance to try your other suggestions. I'm guessing that by now it may be too late.