On MacOS and Windows (I can try this on PopOS as well if you like) FF launches instantly with no complaints if the network is down. Which tells me that FF seems to properly handle situations where things are merely unreachable.
The fact that it's hanging for you implies to me that perhaps FF is getting some kind of malformed response from... something on your network. A firewall, a proxy, I don't know.
I've not tried with those machines on a different
network. I assume it's related to networking in some
way. My network is more complex than most.
I don't know if you're open to feedback on this or not, but it feels disingenuous to state "Firefox is unusably slow" without noting that it appears to be some specific interaction between Firefox and your network. If it is a network issue, it certainly sounds like it is something FF should handle more gracefully/informatively, but that is a different matter from "Firefox unusably slow."