Um, no...
I'm using 10.5.5 on a Mac Pro, and this does not happen.
I enabled FileVault on my Mac Pro for an admin account.
Opened Safari, set FireFox as the default browser.
Saved a URL to the desktop.
Rebooted.
Double-clicked the URL on the desktop,
FireFox opened the page.
So, I guess I call it fixed or bull.
Biting the hand that feeds IT
