In bug 1355924 a new animation for the stop / reload button was implemented (I hope there will be a preference to disable it because that's *really too much* movement in the toolbar and *very distracting* but that's another topic). I don't know if there is a better animation on other operating systems but at least on macOS the button has another size/width during the animation and this looks definitively not intentional, it looks like an error. You can see the wrong looking animation in the attached video.

This video https://www.youtube.com/watch?v=7MiKf8Rqf5w shows the same animation bug on Windows. You can reduce the speed to see it better.

Bug 1355455 will add the new toolbar icons. There shouldn't be any size mismatch after that bug.