The default browser is set by option "browser", in turn set by
  the environment variable R_BROWSER if that is set, otherwise to
  NULL.
  To suppress showing URLs altogether, use the value "false".
Some browsers have required : be replaced by | in file
  paths: others do not accept that.  All seem to accept \ as a
  path separator even though the RFC1738 standard requires /.
To suppress showing URLs altogether, set browser = "false".