For some reason here it does not work as anticipated.
I don't have Opera ...... I do have Konqueror and Firefox and Iron ...... the first two are listed in the drop-down box.
Have you tried a non-default browser?
Konqueror does not launch if selected there.
If I use the manual box Konqueror does launch as expected.
Iron launches also ...... but does not open the URL ...... I get the default page!
I have tried various commands to no avail.
I then notice that the same behaviour is exhibited by Iron when clicking links from anywhere, so the Iron problem is a separate thing.
I can mark the original problem solved ...... it now launches Iron, although for some reason it is not in the drop-down box, and Konqueror does not launch from there anyway, so probably Iron wouldn't even if it was there.