Comment 8 for bug 17081

It appears that it uses the most recently installed browser installed via apt. I generally install Epiphany as a fallback browser for various things, and it naturally supersedes Firefox as default browser (even though I want Firefox as default.)

Installing Dillo will set it as default. Of course, this issue is not confined to Nautilus. It will also improperly trigger on xdg-open, which means that a variety of (properly engineered) programs fail to use the proper browser. I have personally observed it on:

- Hulu Desktop
- Springlobby (springlobby package in universe)
- Pidgin

It's a significant interoperability issue, since there's no reason to assume software will be using the gnome-open method of opening files.