Hi Mike,

Welcome to MSDN Managed Newsgroup.

This is a known issue of ASP.NET postback with event validation enabled,
and this is not directly related to Firefox. Let me explain the root cause
the possible workarounds.

First, let me explain what is event validation in ASP.NET: this is a
security feature that ensures the postback actions only come from events
allowed and created by the server to help prevent spoofed postbacks. This
feature is implemented by having controls register valid events when they
render (as in, during their actual Render() methods). The end result is
that at the bottom of your rendered <form> tag, you'll see something like
this: <input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION"
value="..." />. When a postback occurs, ASP.NET uses the values stored in
this hidden field to ensure that the button you clicked invokes a valid
event. If it's not valid, you get the exception that you've been seeing.

The problem happens specifically when you postback before the
EventValidation field
has been rendered (i.e., before the page is fully loaded). If
EventValidation is enabled (which it is, by default), but ASP.net doesn't
see the hidden field when you postback, you also get the exception. If you
submit a form before it has been entirely rendered, then chances are the
EventValidation field has not yet been rendered, and thus ASP.net cannot
validate your click.

There are many workarounds for the above mentioned problem, one of them is
that disabling postback until the page is fully loaded as you mentioned:

1. Set enableEventValidation to false:

<pages enableEventValidation="false" ...

2. One way around the problem is to mark the form as disabled and then
enable it in script once the load is complete.
Snippet 1:
========
function enableForm() {
   document.getElementById("form").disabled = false;
}
window.onLoad = enableForm();

Snippet 2:
========
function disableElements(elements) {
   for (var i = elements.length - 1; i >= 0; i--) {
       var elmt = elements[i];
       if (!elmt.disabled) {
           elmt.disabled = true;
       }
       else {
           elmt._wasDisabled = true;
       }
   }
}
function disableFormElements() {
   disableElements(_form.getElementsByTagName("INPUT"));
   disableElements(_form.getElementsByTagName("SELECT"));
   disableElements(_form.getElementsByTagName("TEXTAREA"));
   disableElements(_form.getElementsByTagName("BUTTON"));
   disableElements(_form.getElementsByTagName("A"));
}
function enableElements(elements) {
   for (var i = elements.length - 1; i >= 0; i--) {
       var elmt = elements[i];
       if (!elmt._wasDisabled) {
           elmt.disabled = false;
       }
       else {
           elmt._wasDisabled = null;
       }
   }
}
function enableFormElements() {
   enableElements(_form.getElementsByTagName("INPUT"));
   enableElements(_form.getElementsByTagName("SELECT"));
   enableElements(_form.getElementsByTagName("TEXTAREA"));
   enableElements(_form.getElementsByTagName("BUTTON"));
   enableElements(_form.getElementsByTagName("A"));
}
Make sure that the variable _form is set to the ASP.net form on the page.
Then just call enableFormElements() and disableFormElements().

Hope this helps. Please feel free to let me know if there's anything else I
can help. Thanks.

Sincerely,
Walter Wang (wawang@online.microsoft.com, remove 'online.')
Microsoft Online Community Support

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