Asp.net - JavaScript / Dealing With Different Browsers?

my code works fine in JavaScript but doesn't work correctly in Firefox or safari and wondering why. What I'm doing is I have a loop going through each element and depending on the variable inside a text box just want to alert something. Like i said earlier this code works fine in IE. Here is the code below:

Here is an example of text box:

<asp:TextBox ID="txtMac" runat="server" req="yes" errMessage="Mac"/>
for (a = 0; a < theForm.elements.length; a++) {
if (theForm.elements[a].type == "text" && theForm.elements[a].req == "yes") {
alert("Made it here")
}
}