Comment 4 for bug 239378

GNOME still has issues handling browser-related files (for example, even after selecting Opera as the default browser, it keeps opening htm, html and xml files in Firefox), so maybe this one is part of such issues ---with the disadvantage that in this particular case it's not possible to just modify the "Open with" section for a file in Nautilus.

I tried both aMSN and emesene, to check for a similar behavior: aMSN doesn't depend on GNOME defaults, and therefore it works correctly when the browser string is set to "opera $url". emesene, on the other hand, aims to integrate with GNOME, and as a result it behaves in a similar way to Pidgin when trying to open Hotmail (i.e., uses Firefox, even if the default browser is set to Opera).