Today, CONFIG.sDefaultBrowserExe and sDefaultBrowserParams are internal, default to Internet Explorer and they don't seem to be controlled by preferences.
There should be a preference like fiddler.ui.toolbar.BrowserList.Default and fiddler.ui.toolbar.BrowserListDefaultParams that allows override.
https://groups.google.com/forum/#!topic/httpfiddler/yIUeqEtUlOg