User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:66.0) Gecko/20100101 Firefox/66.0
Steps to reproduce:
Bug happens at random with regular use in seemingly any scenario that would involve dragging the mouse (including the scroll bar).
Happens in Windows 10.
The mouse "picks up" a random piece of media (usually an image) from a page that may not even be open anymore (even from a private session) as if I had clicked and dragged on that image, so that I see the ghost follow my mouse pointer. Releasing the mouse more often than not replaces the current webpage with the direct link to that image.
There is an archived reddit thread describing exactly this issue (though some of the replies seem to be different issues)
An additional point is that usually, in the event that the dragged media do not replace the current page, the mouse gets "stuck" as if I were still holding down the mouse button, highlighting text on the page until I click again.
Whatever I was trying to do with the mouse, and nothing else (i.e. moving the scrollbar).