Firefox refuses to display iframe on my https website, can you help me ? [FIXED]

Hi,

Disclaimer : I'm not a professional coder, I just know bits and pieces here and there and try to adapt when I can (mostly because I can't afford to pay someone to take care of it).

Problem : I'm creating a website, and I'd like to include maps of China in iFrames (rather than using Baidu, blocked in the US and UK, or google for obvious reasons). I went on OSM as always, clicked the share option, selected HTML, included the Tag and copy-pasted the whole in the div of my page. The whole thing seems to fit nicely, except I'm on firefox, and it says :

To protect your security, www.openstreetmap.org will not allow Firefox to display the page if another site has embedded it. To see this page, you need to open it in a new window.

So I wonder, is it really OSM blocking it or just Firefox (I heard modern browsers won't allow http-iframes into https-websites ? I don't really undertsand 'cause OSM is in ssl so...) ?

Can someone help me in a very gentle and detailed way (cause if you speak as if I'm an IT engineer I'm not gonna be able to do much) ?

Thank you