Thanks for your fast reply.
On Firefox 52.0.1 (Mac OSX also), images are not hidden on load, so there's some "break" in website loading. IMages appear at first, then disappear, and finally appear in the definitive way of the design. This is not a really big issue for that one. I alos tested on Firefox for Windows 10 and Edge, and here the website is totally stopped on the loading icon. So I only have the background and the loding icon to be displayed in an infinite loop. If I reload the page then I got the same issue as on Firefox Mac OS version.
Thanks for your time.