Windows 10: Weird taskbar icon thumbnails for Firefox

Not sure if this is a browser (latest Firefox) or Windows problem, so I'll describe the issue here.

All my program shortcuts are placed on the taskbar. The problem I'm about to describe and show, only occurs with the Firefox taskbar icon and its preview thumbnails. And will happen whether one or multiple tabs are on the tab bar. When hovering the cursor over FF's tab bar icon, and then hovering (and not clicking) the cursor over a tab's preview thumbnail, FF's window will now show several tiled FF's with the present thumbnails site on top. But, now clicking on that thumbnail, the site will open as a single site...with no evidence of the other apparently opened FF's. This only happens with the FF taskbar icon's thumbnail and not with any of the other icons on the bar.

Here are some captures of this odd occurrence:










:)

 

Taskbar not showing popup icons for multiple windows

If I only have one of a certain type of window open on my taskbar, then when I hover over the icon for it, a thumbnail of the window will pop up as expected. If however, I have 2 or more of the same type of window open on my taskbar, (e.g. 2 File Explorer
windows) then when I hover over the icon, the names of the windows will be displayed rather than the thumbnails of the windows. How do I change this so that the thumbnails appear when I have multiple of the same type of window open and I hover over the icon
on the taskbar?

 

Windows 10 Auto-hide Taskbar and Thumbnails NOT Disappearing

I see many posts, here and elsewhere, complaining that taskbar thumbnails are not visible, disappear too quickly, or how to disable the thumbnails altogether. My problem is on the other side of the fence: For some reason, my taskbar and thumbnails recently
started staying visible, even when I am working on another task in another window.

For instance, suppose I am in a browser, and mouse to the bottom of the display, which causes the taskbar to appear. When I hover the mouse over the Outlook taskbar icon, the Outlook taskbar thumbnail appears. However, when I return to the browser window,
and operate there, the taskbar and Outlook thumbnail remain visible.

Is there a setting somewhere that governs this behavior?