MC 23.0.103
MacOS High Sierra 10.13.3
I can echo the same things Fred1 said.
I can replicate this every time using CMD+tab. I can also say that this did not happen "before." I don't recall what I upgraded from recently, but I know in prior versions this didn't happen. To me it is always the View menu that shows up. And it shows up relative to where my mouse is. If I have my mouse on the far right side of the screen in Firefox, then I CMD+tab to MC, the View menu shows near the clock/ batter/ wifi area. I hit esc, and it goes away. If I go back to Firefox and move my mouse to the left, then CMD+tab, the View menu shows relative to the mouse.
A simple esc makes it go away, but this is certainly new behavior that I never noticed before.