Set Firefox as your default browser to see if that helps:

Tools > Options > Advance > General and CHECK to make Firefox as your default browser.

Fire Fox is set as my default browser.

The following link might give you an idea though it is for Outlook Express. Do you have ZoneAlarm?

http://kb.mozillazine.org/Integrating_Firefox_and_Outlook_Express

Another link you might want to read:

http://support.mozilla.com/en-US/questions/773937

Last suggestion: Changing the default browser in your PC (you might have done so already):

In Windows XP, you can change it as follows

   Start menu
   Set Program Access and Defaults

But I don't know about Windows 7 since I am using XP, sorry.