u/LinuxGamerLife

You'll need niri-float-sticky
https://github.com/probeldev/niri-float-sticky

spawn-at-startup "niri-float-sticky" "-app-id" "org.mozilla.firefox" "-title" "Picture-in-Picture"

And this window rule

window-rule {

match app-id=r#"^org\.mozilla\.firefox$"# title="^Picture-in-Picture$"

open-floating true

default-column-width { fixed 600; }

default-window-height { fixed 350; }

default-floating-position x=300 y=60 relative-to="top-right"

}