I just did a clean install of Ubuntu 18.04 on my laptop, and when I view this forum on Firefox on this computer, images within the posts on this forum are not displayed. I didn’t have this problem with 16.04, nor with Manjaro, OpenSUSE or Windows. And on this computer, I don’t have this problem on another forum powered by Discourse (the Discourse demo forum), nor do I have problems viewing images on other web sites.
All this suggests that there is a setting in the forum that is causing my problem, but I can’t seem to find it. Any clues?