I recently added Opera to my list of browsers on DW CS4.
Now, when I click on IE, Opera comes up.
Opera also comes up when I click on Opera.
Re-define your browser list, making IE your default browser and Opera your secondary browser.
If that doesn't stick, you may need to delete your personal config folder so DW can create a new one for you.
See details below on where to find it:
Retrieving data ...