Page displays random symbols instead of error message on Firefox

This is looks like gZip error decoding.

Check if you set the Content-Length in a way on your pages, and then use gZip filter. If yes then remove the Content-Length set from your code.

This can happend when you send the Content-Length and later you compress it and iis can not change the header Content-Length to the new compressed one, and send the wrong size, then browser is reading the wrong size and fail to decompress it correct.

reference:
ASP.NET site sometimes freezing up and/or showing odd text at top of the page while loading, on load balanced servers

Update

Other possible reason is to set a wronge Response.ContentType, for example to set it as text and you send a gif image, or to set it as image and you send a text.

Update 2

Maybe the error is on the the content type. Set this headers:

context.Response.ContentType = "application/octet-stream";
context.Response.AppendHeader("Content-disposition", "attachment; filename=" + cFileNameToShowAndDownload);

I did not find any real solution, but I found a satisfying workaround.

Keep in mind that the problem only arises under those conditions :

• The website is configured on IIS7 / Windows Server 2008.
• The page displaying the garbage symbols has, in reality, crashed. The resulting "garbage" is, in fact, an gzip-compressed error message that has not been decompressed, or something like that.
• Disabling gzip compression on either dynamic or static content does'nt change anything

The workaround is simple : refuse gzip-compressed content in the browser. In Firefox, as seen in http://forgetmenotes.blogspot.com/2009/05/how-to-disable-gzip-compression-in.html :

1. Type about:config in the URL bar (Accept the disclaimer)
2. Type encoding in the filter field underneath the URL bar
3. Doubleclick the line "network.http.accept-encoding"
4. Empty the value

On my website, it did some weird things with the CSS (and StackOverflow does not have any CSS at all after that), but at least it correctly showed me the error message, which enabled me to fix the bug.

Hopefully it will help someone.

Another workaround for anyone stumbling across this could be to remove the response filter on application error, which will get rid of the encoding and send it through uncompressed. Since this is only for error messages the impact on performance should be minimal.

In your Global.asax

VB

Sub Application_Error()
    Response.Filter = Nothing
End Sub

C# (I assume this is right, see blog link below)

protected void Application_Error(object sender, EventArgs e)
{
    Response.Filter = null;
}

All credit to Rick Strahl with this blog post for the workaround.

Note: I also added this an an answer to my own question here - IISExpress garbled HTTP 500 error message