Put a div around the textbox and write handler in the div's onclick.
<div onclick="txt1OnClick()">
<input type="text" id="txt1" runat="server" disabled="disabled" class="blacktxt" style="width:300px" onclick="txt1OnClick()"/>
</div>
function txt1OnClick()
{
alert('Hello world!');
}
This trick doesnot work in FF :(