If you are using Gnome, then this article may help for updating the Preferred Applications:
Adding an application to the default applications.
It describes the format of an xml file that defines the new application, to be placed in the directory
For stopping the "default browser" dialog, you need to set to False in
about:config the item
browser.shell.checkDefaultBrowser. See this thread for all default files. The one you are looking for is either mozilla.js or prefs.js. Writing Firefox configuration files is not my specialty, but this or this might help.