http://zzzmirror.i2p/topics/234-how-to-select-browser-for-startup-you-can-t
On anything else, it tries in the following order: opera, firefox, mozilla, netscape, konqueror, galeon, links, lynx. It appears that the systray code supports a "systray.config" file, and a string browser=foo in there would open browser foo. This is only for launching the browser from the little icon in the systray (windows only).