Firefox float bug? How do I get my float:right on the same line?

Change the order of the float, put it before the text like this:

<div class="btn-group">
  <button data-toggle="dropdown" class="btn btn-default dropdown-toggle"style="width: 400px;text-align: left;">        
    <span class="glyphicon glyphicon-chevron-down" style="float: right;"></span>
    Checked option
  </button>
</div>

http://jsfiddle.net/q05n5v4c/3/