You can already handle the problem 3 different ways:

1. global override, no condition:

$.ajaxSetup( {
  xhr: function() {
    return new window.XMLHttpRequest( {
      mozSystem: true
    } );
  }
} );

2. local override, simple condition:

$.ajaxPrefilter( function( options ) {
  if ( options.firefoxOS ) {
    options.xhr = function() {
      return new window.XMLHttpRequest( {
        mozSystem: true
      } );
    }
  }
} );

// Then later on

$.ajaxSetup( {
  firefoxOS: true
} );

3. Generic constructor parameter

$.ajaxPrefilter( function( options ) {
  if ( options.xhrConstructParam ) {
    options.xhr = function() {
      return new window.XMLHttpRequest( options.xhrConstructParam );
    }
  }
} );

// Then later on

$.ajaxSetup( {
  xhrConstructParam: {
    mozSystem: true
  }
} );

1. is simpler but cannot be used generically in a project that may run outside of FirefoxOS,

2. is better at isolating FirefoxOS specific code,

3. is to be used for a more generic approach.

Anyway, we should probably consider adding a standard option (as in 3) seeing as the options object param is part of the standard now: http://www.w3.org/TR/XMLHttpRequest/#dom-xmlhttprequest

Method 1 seems good enough here? If we're going to add more code we need docs before landing it, and I shudder to think of more docs for ajax.

Several good solutions were given above so it seems like we can avoid another mechanism and the docs it would require.