By default Auto Complete feature is enabled in all browsers.To disable auto complete use AutoCompleteType poperty of text box.
<asp:TextBox id="txtname" runat="server" AutoCompleteType="disabled"></asp:TextBox> or
<asp:TextBox id="txtname" runat="server" AutoCompleteType="none"></asp:TextBox>
From code behind: txtname.AutoCompleteType=AutoCompleteType.Disabled
or txtname.AutoCompleteType=AutoCompleteType.None
This work in IE but not work in FireFox.So,for FireFox use autocomplete property.
<asp:TextBox id="txtname" runat="server" autocomplete="off"></asp:TextBox>
From code behind: txtname.Attributes.Add("autocomplete", "off")
Also,AutoCompleteType creates a list of values entered in a text box. When using the text box again, a list of previously entered value is displayed.When this property is enable text box have a yellow background on loading by default.
<asp:TextBox id="txtname" runat="server" AutoCompleteType="FirstName" runat="server"/>