Hi Neil,
From my understanding, what you want to do is setting Firefox as the
default browser for debugging in VS.net 2003, right?
If so, you can right-click a page in VS.net 2003 and select 'Browse With'.
In the dialog, you can add Firefox browser and set it as the default one.
After this, VS.net will lauch Firefox when starting debugging.
Please let me know if there is any further issue. Thanks.
Best regards,
WenJun Zhang
Microsoft Online Partner Support
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