if firefox is the default browser, I guess just start "" "https://your.url.here.foo", as per https://superuser.com/a/36730
otherwise, I guess you could just cd to firefox's directory and do the same firefox.exe urlhere
as for specific window... yea that might be tad hard. https://wiki.mozilla.org/Firefox/CommandLineOptions doesn't seem to have any way to indicate any specific instance/window from cli.
could be firefox handles those internally, kinda seems like urls open up in the window which was last active. So.. I guess you could start the script by starting firefox with --new-instance or --new-window, and patiently wait until urls are open? I guess.