Since Gnome window buttons show, there is no minimise or fullscreen button. Window will still fullscreen with a doubleclick on the titlebar, and minimise by the taskbar. Experienced in a clean install of Kubuntu 23.10 on HP EliteBook 840 G2, fully updated. Fault been noticed plenty of times prior, on other hardware. Fault does not demonstrate in firefox on X11. Fault does not demonstrate in the Firefox → help → about window! (I believe I have seen it on other Snap apps under KDE and Wayland, but can’t replicate now, other apps may have been fixed).
WORK-AROUND. Customise toolbar in firefox, enable titlebar. Consequence: Lose screen-space to the titlebar.