I'm thinking he runs a script that opens the browser as a window minimised at startup. So any browser window he opens is just another window and Firefox doesn't have to startup every time
Glad that's working for you but for me Flatpak is noticeably slower at startup (last couple distros, like uhh Fedora, Cachy, OpenSUSE). This is not an indictment of Flatpak in general; it's a nice project. But not all of us can ruin the distributed supercomputers you're running in your laptop