You can open the Web Console (Firefox/Tools > Web Developer). Check on the Network tab if any content is blocked and has a file size 0 (zero).

• https://developer.mozilla.org/Tools/Web_Console

A red line appears above the console with the text, "Style sheet could not be loaded. https://developer.cdn.mozilla.net/media/css/mdn-min.css?build=cbde7a6"

Can you open this file via the location/address bar?

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Yes. I'm prompted about a security exception but, once I click through it, the page appears.

What kind of security error message did you get?

Can you attach a screenshot?

• http://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

• Use a compressed image type like PNG or JPG to save the screenshot
• Make sure that you do not exceed the maximum size of 1 MB

Here's the security dialog I got trying to go to the style sheet directly. I get the same originally going to the parent page. I suspect this is because the proxy is substituting its own certificate. But having accepted that on the parent page, I don't get an opportunity to accept it for all the constituent pieces.

I don't see the image I asked to upload, snapshot1.jpg. That could be the result of this problem; I'm working from the page without any style. I click browse and select my file, the name appears next to the browse button, but there is no explicit way to upload it. I previously guessed that it would upload along with my note but I sure don't see it. Trying again.

• Use a compressed image type like PNG or JPG to save the screenshot
• Make sure that you do not exceed the maximum size of 1 MB

Hey, I already said the file is a JPG. The size is 100 KB, far below any limit.

The picture is the standard, "This connection is untrusted", (poor grammar and all) with the traffic cop icon.

What does it say in the technical details on that error page and who is the issuer of the certificate?

You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates.

• Click the link at the bottom of the error page: "I Understand the Risks"
• Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate"
• Click the "View" button and inspect the certificate and check who is the issuer.

You can see more details like the intermediate certificates that are used in the Details tab.

developer.cdn.mozilla.net uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. (Error code: sec_error_unknown_issuer) -- Issued to Common name: *.cdn.mozilla.net .. Serial number 46:8B:B1:35:00:00:00:00

Issued by Common name: proxy2.mycompany Organization Internet Widgits Pty Ltd ..

If you want to manually add an exception for the style sheets on the https://developer.cdn.mozilla.net/ server, visit it directly.

Or, assuming you trust all certificates signed by that issuer --

Common name: proxy2.mycompany 
Organization Internet Widgits Pty Ltd

-- you may be able to import its signing certificate into the Authorities section of the Certificates dialog so that you no longer encounter this problem. I am not sure of the steps involved.