Not sure how Firefox code is structured but it is weird that a particular HTTP3 request would hang the entire network stack and you cannot perform any HTTP 2/1 requests.
You know what happens when you put a while(1); in the middle of the nginx codebase? The whole server process hangs. This is normal in an async design. We don't write software to be magically resilient against freak bugs, especially not something like a browser that is not intended to be used in life-critical applications.