Windows startup Powershell script not closing after Start-Process

I've got a Powershell V2.0 startup script for my work computer (XP Professional 64-bit), as follows:

start "C:\Program Files (x86)\Microsoft Office\Office12\OUTLOOK.EXE" -ArgumentList "/recycle"
sleep -S 2
start "C:\Program Files (x86)\Mozilla Firefox\firefox.exe" -ArgumentList "https://mail.google.com"
sleep -S 2
start "C:\Program Files (x86)\Mozilla Firefox\firefox.exe" -ArgumentList "-new-window https://www.google.com/calendar"
sleep -S 2
start "C:\Program Files (x86)\Skype\Phone\Skype.exe"

The sleeps are to ensure that the windows appear on the taskbar in the correct order. I run this from a shortcut on my Quick Launch with the following Target:

C:\WINDOWS\system32\WindowsPowerShell\v1.0\powershell.exe C:\scripts\initialize.ps1

(Yes, this is 2.0: powershell -Version 2.0 works, as does -Version 1.0, but not -Version 3.0)

Problem is, the command window stays open until the Firefox windows are closed, which is not what I want. Looking at Process Explorer when I run the script, here's what happens:

1. powershell.exe appears under explorer.exe and the Powershell window appears (with a black background, oddly. But it's not cmd.exe, since when I was debugging the script error messages would appear in red).
2. outlook.exe appears under powershell.exe and the Outlook window appears.
3. firefox.exe appears under powershell.exe and a Firefox window appears.
4. A second firefox.exe appears under powershell.exe and another Firefox window appears. The second Firefox process then exits, as expected, since Firefox only uses one process.
5. skype.exe appears under powershell.exe and the Skype window appears.
6. The powershell.exe process inexplicably sticks around, as does the Powershell window.

If I close both Firefox windows, the powershell.exe process exits and the Powershell window closes, and the outlook.exe and skype.exe processes appear under explorer.exe as expected.

I suspect this has something to do with Firefox's standard input, output and error: I wouldn't expect Outlook or Skype to ever output anything to the console, but Firefox has command-line options that allow it to do so. I've looked over my about:config's user set values and didn't find anything suspicious. Finally, if I have a firefox.exe instance already running (started from the desktop shortcut) the problem doesn't occur (the powershell.exe process exits as it ought to).

So what's going on here? I'm going to try adding -WindowStyle hidden to the shortcut next (gotta close this Firefox to test it), but I want to get to the bottom of this, if only to improve my understanding of how Windows consoles work.