I can reproduce the issue in KUbuntu's 24.04 LiveCD, with FF DevEd 151.bX and both the Oxygen and Breeze window decorations. Haven't tried others. https://discuss.kde.org/t/kwin-causing-firefox-windows-to-be-downsized-upon-restart-with-the-browsers-session-restore-enabled/46785/11?u=rjvbb