If you check your History menu (either using the "3-bar" menu button of the classic Menu Bar, if you display that), is Restore Previous Session available? You might also check whether there are any closed windows available to restore.

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

By default, pages that open automatically should open in a new tab and not replace the contents of an existing tab.

Or if you switched off the tabbed browsing setting in the Options dialog, it might open in a new window instead.

Firefox does have a hidden setting that allows you to load external links in an existing tab, but most likely you haven't set that, and I'm not sure it would affect where add-on pages open.