The latest version of Firefox (115) always launches at fullscreen size in linux. I rarely use any application at full screensize because I like to have other windows open on the desktop at the same time. Virtual desktops are not always convenient. I have tried using the -geometry option at the command line, which is usual honoured by applications running under X11, but this is ignored. Nor can I grab the bottom right-hand corner and drag it diagonally to resize it dynamically. Maybe there is a setting somewhere for initial size?