Consider this a BUG report then, and this is how it looks to me.
Disabling the “Web Shield” appears to resolve this problem. Mozilla developers claim that Chrome blocks HTTP/3 proxies via AV programs to prevent this problem, and Mozilla is considering doing that too.
I wasn’t familiar with HTTP/3 and QUIC. I’m not very familiar with using proxies. I don’t know all the programming issues relating to this problem. So I cannot suggest a specific coding solution.
But the symptoms are very slow responses, long wait times loading specific web pages (Amazon, Youtube, Sourceforge), and Youtube videos halting (also video resolution downgrades) when Avast web shield is active and when using Mozilla Firefox 130 or higher. If you try the same web pages in Chrome, Brave, LibreWold browser then performance appears normal. Disabling Avast Free “Web Shield” restores normal performance in Firefox. Disabling Web Shield is a good workaround, but I might be losing some protection by not having it.