One of these things is not like the other -- Chrome vs. Firefox security
The attached image shows Chrome and Firefox accessing the exact same URL at the exact same time. Chrome is successful, Firefox is not (actually, fails over and over). So . . . wut? Clearly not a network problem, clearly not an accessibility problem, there is no firewall or proxy and Firefox is able to access other sites. Only this one is a problem. At least, so far. Security setting is Standard.
Alle svar (1)
Did the site's home page (https://nighttime-imaging.eu/) load for you?
That site seems to be working normally for me at the moment (in case it's relevant, I sit in California).
I'm not sure exactly what to make of the error page. It doesn't say server not found, so presumably Firefox did find an address for the site, but maybe it's incorrect? You could experiment with the DNS over HTTPS setting to see whether changing your DNS service affects your ability to use the site. This article will get you to that setting: Firefox DNS-over-HTTPS.
That same Network Settings dialog is also where you could set "No proxy" just in case there is something in your system settings that Firefox is trying to use.