Android: Firefox shows "Proxy Server Refused Connection" when using VPN

EDIT2:

This is fixed in Firefox beta and nightly. See https://bugzilla.mozilla.org/show_bug.cgi?id=1858377 for details.

The patch hasn't been backported to release channel yet.


EDIT:

Ticket filed at: https://bugzilla.mozilla.org/show_bug.cgi?id=1857810

It seems to related to how Firefox (incorrectly?) handles system proxy, which my VPN app might add one. Not sure if related to Android version; I'm using 13 FWIW.


I often use VPN on my phone for certain circumstances. I've been using the same VPN for years, and it worked fine with Firefox.

Recently (not sure when exactly), I found that as soon as I enabled VPN, Firefox failed to connect to the Internet.

Every page shows "Proxy Server Refused Connection".

Some more info:

  1. I have zero network issues with any other apps. Firefox is the only app having this issue.
  2. I've tried Beta, Nightly, no difference.
  3. I've tried clean all the data, reinstall, etc., no difference.
  4. It still works on my backup phone, with exactly same VPN and same settings.

Something interesting: the home page of Firefox somehow still has Internet access as it can load these Pocket "stories" just fine. But as soon as I started to open any website, it immediately shows "Proxy Server Refused Connection".

I wish to debug further but I don't know how. Unlike desktop version, there is no proxy setting in Firefox Android.

What should I do?

Thanks in advance.