Setup

Due to firewall restrictions I'm using the "Use Browser to download Meta Data" feature.

This allowed me in the past to view all updates available for jenkins, even though the server itself has no internet access.

Issue

Since the last update to 2.77 this feature stopped working.
When opening the browser console I get the following error:

Mixed Content: The page at 'https://xxxx/sbjenkins/pluginManager/checkUpdates' was loaded over HTTPS, but requested an insecure resource 'http://updates.jenkins-ci.org/update-center.json.html?id=default&version=2.77'. This request has been blocked; the content must be served over HTTPS.