Private windows: mask and hamburger no longer overlap
Firefox puts its private-browsing mask button at the end of the tab strip, right where FoxOne pins the hamburger menu. In a private window the two sat on top of each other. The mask now takes the hamburger clearance and slides in next to it, so the strip reads extensions, mask, hamburger, window controls.
Private windows keep Firefox's own purple palette as before. FoxOne hands off to the built-in theme there on purpose, so the window state stays recognisable at a glance!