Screenshot of the issue....?

Hi Mary, what thumbnails are you thinking of? The two that come to mind are:

(1) When you hover the mouse pointer over a background/inactive tab on the tab bar, Firefox shows a thumbnail of that tab. There is a checkbox for this on the Settings page, "Show an image preview when you hover on a tab".

(2)When you hover the mouse pointer over the Firefox button on the Windows Taskbar, Windows shows thumbnails of your open Firefox windows -- up to a certain point: if there are too many, you get a vertical list of titles of the active tabs in each window.

Was it one of those? Something on a website?