Trouble setting applications as "default"

[dapperdanman1400]

I just got a new MacBook Pro with Leopard, and I've installed some custom software to handle things like instant messaging, web browsing, and email. Naturally, when I first opened them, they asked me if I would like to set them as the default application for the task that they would be performing. However, when I restart my computer and launch the programs, they ask me once again if I want to set them as the default. This only happens when I start up the program for the first time after the computer is turned on. It doesn't ask me again if I close the program and start it again without restarting the computer. Here are the programs that this problem is happening with (they are all the latest version):

-Firefox
-Thunderbird
-Adium