browse-url doesn't support firefox

[Dave Love]

There's no browse-url support for firefox.  I guess it should be
documented that it works to set `browse-url-mozilla-program' to
`mozilla-firefox', at least.  Presumably it be detected for use with
`browse-url-default-browser'.

Also `browse-url-default-browser' should look for `sensible-browser'
at high priority, which will DTRT on Debian.

There should be a TODO item to clean up the mess in browse-url and use
a shell command string to specify the browser instead of the
mushrooming set of functions.  See also ESR's proposal for a BROWSER
environment variable <URL:http://www.catb.org/~esr/BROWSER/browse-url.patch>.