Hello,

Try Firefox Safe Mode to see if the problem goes away. Firefox Safe Mode is a troubleshooting mode that temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes).

If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:

• Click the menu button , click Help and select Restart with Add-ons Disabled.

If Firefox is not running, you can start Firefox in Safe Mode as follows:

• On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
• On Mac: Hold the option key while starting Firefox.
• On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
  (you may need to specify the Firefox installation path e.g. /usr/lib/firefox)

When the Firefox Safe Mode window appears, select "Start in Safe Mode".

If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.

To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.

When you figure out what's causing your issues, please let us know. It might help others with the same problem.

I'm sad to say that trying Safe Mode made no difference. The duplicate tabs/pages persisted there. Going back to regular mode also didn't make a difference.

The only extensions/themes I have running are for LastPass and Light. Both of which I've used with no problems for years.

If I uninstall and reinstalll Firefox, might that help? Would I lose all my data (bookmarks, sync settings)?

I tried the Refresh Firefox option. It did nothing to eliminating the problem. Are there any suggestions before I delete FF altogether?

What security software do you have?

Boot the computer in Windows Safe mode with network support to see if that has effect in case security software is causing problems.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.7tutorials.com/4-ways-boot-safe-mode-windows-10

Hi juliet.drennan, this is a very puzzling problem.

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.)

One other note: On Windows, 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":

You can check for problems with the sessionstore.jsonlz4 file in the Firefox profile folder that stores session data.

• delete sessionstore.jsonlz4 and sessionstore.js

When sessionstore.jsonlz4 is deleted you lose pinned tabs and open tabs and you will have to recreate them, so if necessary make a note or bookmark them.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox