This is the demo http://jsbin.com/ogiyej/9/edit
When I choose the first option to 'a',
then choose the second option to 'a1'
the third option will be changed into several options like "a11 a12 a13" etc.
But when I change the first option to 'b'
then choose the second option to 'b1'
in firefox the third option changed to the 'b1''s district very slowly;
chrome work well.
Both browsers were updated to the latest version.
(In firefox if I click the second option several times,ui will be updated soon)