I was browsing Firefox, and I selected the Help and Tools pulldown menus in quick succession, but since I moved the mouse to Tools before Help fully loaded (race conditon?), the Unity highlight shadow that surrounded the pulldown menu (before it fully expanded to width) got stuck on somehow. It persists above all other windows, it's not clickable, not selectable, and blocks clicks to whatever is behind it.

What is this? How do I kill it, preferably without logging out and back in?

I have restarted Firefox already. Fullscreen text editor screenshot to show the thing more clearly. The only thing that gets "in front of" this is the Dash, but you can still see a blurred rendition of the stuck highlight shadow zombie lurking "behind" the Dash. (Ubuntu 16.04 LTS, but this issue may not be version-specific)