Also, I do have an NVIDIA card and gfx.webrender.allqualified = True.

https://support.mozilla.org/en-US/kb/changes-toolbars-and-window-sizes-are-not-saved You may have corrupt xulstore.json file.

Close Firefox. Open your file browser to this hidden location:

• Mac: *~/Library/Application Support/Firefox/Profiles
• Linux: *~/.mozilla/firefox/Profiles
• Windows: %AppData%\Mozilla\Firefox\Profiles
• *C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\Profiles
• Windows XP: C:\Documents and Settings\<user>\Application Data
  \Mozilla\Firefox\Profiles\

Search for the above file. Then rename or delete it. Restart Firefox.

I tried that before I posted, to no avail. Other ideas?

Can you open the system menu via Alt+Space?

You can cZheck that you do not run Firefox in compatibility mode. You can open the Properties of the Firefox desktop shortcut via the right-click context menu and check the "Compatibility" tab. Make sure that all items are deselected in the "Compatibility" tab of the Properties window.

Compatibility properties are all unchecked.

I have determined more context: The situation occurs when I close Firefox from my auxiliary monitor, and only after I have opened more than one tab and actually loaded a sites in them. I am doing further assessment to figure out exactly when the problem does and does not occur. It is not consistent anymore, so I am doing more investigation.

Running an NVIDIA GeForce 940MX card. It also appears that an Intel graphics driver is in play in addition to the NVIDIA graphics. Should it be? It seems like my computer should just be using one or the other, but perhaps it auto-optimizes which to use based on the context? I bring up the graphics drivers because of the new WebRendering Firefox is (should be) using in this new version. Reinstalling Firefox and turning off WebRendering are on my list of steps to try.

You can try to disable hardware acceleration in Firefox.

• Options/Preferences -> 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 for changes to take effect.

• 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

After the above, open Firefox normally. Once open, minimize that window. Right-click on the Firefox icon and select Open New Window.

Does this open a normal window? If it does, close the first window. Wait a few seconds, then close Firefox.