Does it still happen in Troubleshoot Mode?

I don't know. I looked at running in Troubleshoot Mode using your link. I went as far as the help selection, but didn't choose troubleshoot because I was already beyond my comfort zone and afraid I might do something that would cause more problems than I have already. If I do restart in troubleshoot mode can I exit after checking a document display on the monitor using Firefox? I looked at the if/then statements after restarting in troubleshoot and I am not that tech savvy with this to try that.

Try to start Firefox in Troubleshoot Mode to see if that works.

• Windows: hold down the Shift key while starting Firefox
• Mac: hold down the Options key while starting Firefox

You can try to disable hardware acceleration in Firefox.

• Settings -> General -> Performance
  remove checkmark: [ ] "Use recommended performance settings"
  remove checkmark: [ ] "Use hardware acceleration when available"
• https://support.mozilla.org/en-US/kb/performance-settings

Close and restart Firefox after modifying the setting to apply changes.

• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

You can check if there is an update for your graphics display driver and check for hardware acceleration related issues.

• https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

I took a printout of these suggestions to Geek Squad along with my new pc, my external monitor, and apparatus since all of this is above my pay scale. Tech's exact word's "You don't want to be trying to mess with this. This is working with the regiesters" right before he said that they would not do anything with changing software.

So the answer is after 20 years or so with Mozilla I am unfortunately forced to move to another browser or give up my monitor. So Chrome it is.

Does it still happen with hardware acceleration disabled?

zero knight - I have no idea what you're referring to. If it is in trouble shooting that's above my pay grade and geek squad tech advised me to stay out of trouble shooting.

starshipraleigh they are giving you the help that is the basic starting point. You have to do this if you want to narrow down the issues. Trouble shooting is the starting point here to find where the problem is starting from. Geek Squad is pretty useless here. And since you give no PC specs or monitor specs no one can verify what the components are doing with dual display or if the chipsets can handle dual display output. And no two problems are the same unless the hardware and setup matches each other to compare to find the issues at hand.

I've put all of the information that I have in here. It's a Dell monitor, not a new one, manufactured in 2011. It's a new pc manufactured last year or this year - also Dell replacing 3 year old pc that had no problem with this monitor. not dual display if you mean 2 monitors. 1 monitor. no clue what a chipset is. I have solved my issue. I have changed my browser to Chrome slowly but surely after purchasing 3 new pc's (one lenovo Yoga which I didn't like, but which actually worked and 2 dell), a hdmi to vga cord, a new monitor (Dell). So all but one Dell pc have been returned for refund since they didn't solve the issue. Perhaps I'm being naive to think that when I replace a pc in a simple set up that the browser should be able to handle it. It's handled every pc I've purchased over the last several years - and Chrome and Edge are apparently fine with it also. So basically I set this up to let Mozilla know there's an issue with the browser not handling at least two of the new Dell laptops if they're being used with an external monitor. I'm almost totally off Mozilla. Yes, Geek Squad is pretty useless here and that's what the tech told me. They don't mess with fixing software and neither do I.

First one is at original pc display setting 1920 X 1200

Second one is at 1920 X 1080 to match the setting on the monitor.