Bug report: Kaiteki always ignores login on Floorp/Firefox

[leap0x7b]

What happened?

For some reason, if I try to login on Kaiteki Web on Floorp, it would straight up doesn't work and always kicks you out to the welcome screen after you authorized the OAuth request.

Expected behavior

Should login fine and brings me to the home screen.

How to reproduce

1. Use Floorp (or maybe any other Firefox-based browsers)
2. Go to web.kaiteki.app
3. Login to an instance like labyrinth.zone or wetdry.world

What platform are you seeing the issue on?

Web

Additional context

I previously used Kaiteki on Firefox Nightly mobile and it works just fine. It probably affects just Floorp and not Firefox.