Ok, felt guilty for turning this thread horribly complicated with my earlier mention of hidden windows and then the app I suggested failing to deliver

, so I did a bit of scouting and may have found a sort of solution, ditching runhide and instead using nircmd.exe from
http://www.nirsoft.net/utils/nircmd.html
It's not perfect, but you only see a popup dosbox for a split second before Skace's batch executes and does exactly as he intended it to do.
First create the batch file Skace put together >
@echo off
taskkill /f /im firefox.exe
call "C:\Program Files\Windows Media Player\wmplayer.exe"
start /d"C:\Program Files\Mozilla Firefox" firefox.exe
goto :eof
I stuck with the earlier suggested name of bleh.bat, cos I wasn't feeling particualrly creative, so plagiarised the earlier suggestion
Then I created another batch file in the same folder, containing the following
nircmd.exe exec hide bleh.bat
I called this hopeful.bat cos fingerscrossedthis works.bat was a tad unweildy
Now if you click on hopeful.bat you'll see a dos window popup for a fraction of a second, then firefox closes, windows media player opens, and when you close windows media player, firefox re-opens as programmed by Skace
Like I say, it's not perfect, but the dosbox that does popup is literally gone in a blink, so it may be workable for your needs
Edit: By the way, if you have the session saver plus extension installed in firefox, it gives you the option upon firefox restarting of restoring the active session at the time the batch closed it. So if you had 5 tabs and a half written reply penned on anandtech, it all pops up exactly as it was when firefox closed, which could be handy. Saved me a ton of retyping while testing