classList() method which provides the behaviour you would expect from a method implementation of the HTML5 classList property which is supported currently in FF 3.6

jQuery.classList.js

/*
Usages: 
$(selector).classList() //returns an array of classnames
$(selector).classList('newclass') //replaces the current element's classes
$(selector).classList('new', 'class', 'names') //replaces the current element's classes
$(selector).classList(['new', 'class', 'names']) //replaces the current element's classes
*/

jQuery.fn.extend({
  classList: function( value ) {
    if ( jQuery.isArray( value ) ) {
      // An array was passed, join it into a string.
      value = value.join(' ')

    } else if ( value ) {
      // A non array was passed (either a sing string or multiple stings),
      // join them into a string single string.
      value = Array.prototype.join.call( arguments, ' ' );
    } else {
      // No arguments were passed, return an array of class names.
      return this.attr( 'class' ).split( /\s+/ );
    }

    // Set class names and return the original jQuery object.
    return this.attr( 'class', value );
  }
});

You can check for native classList support as follows:

if (!!document.createElement('p').classList) {
 // native classList support
};

It could be a good idea to add this feature test to your plugin and just map $.fn.classList to the native implementation if it’s available.

The thing is, that classList is a token list property of the html element and this is a method of the jQuery collection, so mapping the method to the native implementation might not make sense. But i'm going to think about it.

You’re right, I should’ve looked more closely. Your jQuery classList does other stuff than HTML5 classList.