Potential new proxy leak vulnerability in Firefox
This description sounds like an inducible proxy leak to me:
https://blog.mozilla.org/security/2021/10/25/securing-the-proxy-api-for-firefox-add-ons/
It sounds like all an attacker needs to do is block a proxy connection and the browser will bypass the proxy. It seems important to make sure that this failover functionality is disabled in Tor Browser.