Hi monger2k6, this is a very odd problem!!

First, I know it's a cliche, but have you tried a normal Windows shutdown and restart to release file locks and complete any pending system updates?

Next, I don't think Firefox has a feature to duplicate a window, so it's difficult to understand how that is happening.

How are you opening a new tab? For example:

• Clicking the + button on the tabs bar
• Ctrl+t
• right-click a link > Open Link in New Tab
• Ctrl+click a link
• "middle-click" a link using the mouse scroll wheel
• some other method

Could you show the two Firefox windows side-by-side? Here's what I mean. With one Firefox window active/focused, hold down the "Windows" key and tap the left arrow key. This should tile that window to the left side of the screen. Windows then will hopefully show thumbnail images of all the other open windows. Click the other Firefox window to tile it to the right side of the screen.

Here's my question: are they synchronized/mirrored? For example, if you switch between the tabs in one window, do they switch in the other window? If you scroll down the page in one window, does the other scroll automatically? (Firefox doesn't have anything like that built-in, as far as I know, but it could be some new thing in Windows or some other program.)

Could you check your home page setting? If for some weird reason every tab you open gets added to your home page setting, they would all open in each new window you open. You can find that setting on the Options page. This article has more info: How to set the home page.

I use "+" to open a new tab, plus the "Open Link in New Tab" options. It happens with both. I open a new tab, a second window appears with the two tabs in each window. I happens with each new tab opened. New window, new tab an exact duplicate of the previous windows. If I close a tab, that new window closes, but that tab also closes on the original window.

 I have tried doing screen shots, but the individual windows act like tabs.  They only show on the taskbar, and don't appear side by side.  It is the strangest thing. But when I highlight them, it's identical windows with identical tabs.   You'll see at the bottom that Firefox has two windows.  The picture below is a screenshot of each window separately.  It still acts like one window, that's the strange part.

Okay, now that you mention Taskbar, there is a setting I'd like you to check:

Firefox has two different possible displays for the Taskbar:

• One tile/thumbnail per window, showing the active tab in that window
• One tile/thumbnail for every tab in every window

The setting for this is on the Options page, General panel -- uncheck the box for "Show tab previews in the Windows taskbar":

Was that it?

That did it. I have no idea how that tab got checked in the first place. I feel a little dumb for missing it, but appreciate the help. Thanks

I think someone else had lightning strike in the same way this past week. I have no idea how it happened.