I had a weird issue in Firefox where any Youtube link redirected to HTTPS. I thought initially it was the youtube.com site that was doing it – but this article suggested Firefox was the problem: https://support.mozilla.org/en-US/questions/963015
Now for most cases this would be a good security feature – but the problem with HTTPS is that it masks the HTTP URL from a Proxy Server that may want to inspect or log every web page visited.
Its detailed here: HTTP Strict Transport Security (or ForceTLS).
It turned out the fix was to open Firefox History and make Firefox “Forget About This Site”
The other alternative is to do a “man in the Middle” inspection of the HTTPS connection on the Proxy Server – but this requires issuing all clients with the Proxy server’s Certificate as a trusted Certificate Authority.