I came across a helpful substitute! (I asked on Stack Exchange.)
It's possible to have Steam open a browser while thinking it's opening the game.
This is done using the Properties -> Launch Options dialogue.
I use Waterfox, but this should work for Firefox too.
"C:\Program Files\Waterfox\Waterfox.exe" https://screeps.com/a/ -start-debugging-server %command%
Normally, Steam puts the game's exe at the beginning of the command. But if you place %command% anywhere, it replaces that with the path of the executable instead.
The trick is to put %command% after a flag that won't do anything you don't want it to do. If you just put it at the end without such a flag, it considers the game's path to be an url and opens two tabs, one for screeps and one to try to download the game's exe from your own computer.
