oh man, you basically are my hero. I just found out https://www.trullala.de/firefox-http2-ipv6-pitfall/ which basically describes the problem.

basically we have a gitlab instance at IP .8 / external at .210 and a nexus instances at .210 internally/externally however we have IPv6 addresses pointed at gitlab.

In firefox sometimes you could end in the wrong location and I had no idea why that was happening. it just failed. And only in firefox.

(btw. the behavior of firefox is just stupid. https://bugzilla.mozilla.org/show_bug.cgi?id=1190136)