TextBox doesn't fire TextChanged Event on firefox, AutoPostback is true

i have a issue with my asp textbox not firing textchange event in firefox correctly ,as it works fine with chrome/IE and i know the cause as it is the javascript onkeyup event , if i remove this event the code works fine ;but i can not remove the onkeyup javascript code as it is essential for form validation; here is the code:
<asp:TextBox ID="_tbsname" runat="server" CssClass="round" Width="80%" onkeyup="leftTrim(this);" onblur="this.onchange();" AutoPostBack="True" ontextchanged="_tbsname_TextChanged"></asp:TextBox> <asp:FilteredTextBoxExtender ID="_tbsname_FilteredTextBoxExtender" runat="server" Enabled="True" TargetControlID="_tbsname" InvalidChars="12345667890abcdefghijklmnopqrstuvwxyz,./;'\[]=&lt;&gt;?:&quot;{}|+" ValidChars="A B C D E F G H I J K L M N O P Q R S T U V W X Y Z " FilterType="Custom" ...