So, the question is now:
Does the DNS prefetching of FF work and as a matter of fact there are no (additional) DNS lookup times or where do these times come from?
It is going to depend - the look-ahead parser needs to kick in and then it depends on how it prioritizes the pre-resolves vs other network fetches (all browsers have it these days) and only applies to resources in the html (not javascript). If you look at request 68 you’ll see the DNS resolution happened a fair bit before the actual request so it is working, just maybe not as effective as you might hope.
Sorry, the browsers scan the HTML content and should do a pre-resolve for anything that they can discover while scanning the html (js resources included). They can’t see any domains that are a result of js code executing though (like anything that happens as a result of including the twitter code for example).
I just redid the test with the same server in Germany (that might have some problems with DNS lookups) and it’s clear to see now that it is doing DNS prefetching: