For some reason the default browser is not taken from the OS and it is buried in DOORS configuration.

Having searched for this a few times, I figured I put it out there.

 Tools > Options
 Settings
 Web Browser

Reference:

• http://publib.boulder.ibm.com/infocenter/rsdp/v1r0m0/index.jsp?topic=/com.ibm.help.download.doors.doc/pdf92/ratlDOORS_9-2-0-3_Known_Problems.htm