@RGJ Thanks for the tip.
So I duly create a 144×144 px icon locally (the red square with some text) and add it to Settings > Required > apple touch icon.
So far so good although I don’t have an iPhone to test its use. But the Firefox URL bar warning icon persists. So I check the external URLs using View Page Info and find that Discourse has used http and not https. As shown:
In relation to this URL:
So is the use of http in this situation a bug in Discourse? Or is this correct behavior? Thanks again to the devs and TIA also.
This is not the only custom site-specific icon that is http if you look closely at the screenshot but it will quite usefully serve as our test case. The others were loaded at the time the site was established a couple of years back so they are not so informative.