Client-side event handler doesn't work in Firefox

[vvvoloshin]

Hi, I'm trying to add client-side event handler to my tree. So I have
such code:

string clickHandler = "IndexChanged();";
this.TreeView1.Attributes.Add( "onselectedindexchange",
clickHandler );

It works perfect in IE, but doesn't work in Firefox: Instead of
client-side IndexChanged() execution I have another postback to server
in a case of FIREFOX only.
Is there a way to fix it ?
Thank you,
/Sevan