I have a 2 monitor setup- for the purposes of explaining monitor 1 is on the left and monitor 2 is on the right. When I run Firefox in split screen mode (not maximized) on the right side of monitor 1, the window bleeds over the seem and onto monitor 2. If you look at the attached screenshot you can see a couple pixels on the left edge; it's small but annoying nonetheless and wasn't a problem until a couple weeks ago.

This only happens when Firefox is not maximized and I have not noticed this occurring with any other applications, so it doesn't appear to be an issue with windows or my display configuration.