core: Can't login with Firefox

Home Assistant release with the issue: 0.80.1

Last working Home Assistant release (if known): Don’t know

Operating environment (Hass.io/Docker/Windows/etc.):

Manual installation on a Raspberry Pi 3, running as a daemon

Component/platform: Auth/Login

Description of problem: I just switched from Chrome to Firefox for several reasons. I opened my local home assistant instance and tried to login using the new auth system (with username and pwd). As soon as I click “next” the following message appears: Error: Message format incorrect: required key not provided @ data['client_id']

It looks like this:

I have 2FA enabled if thats relevant. On my PC I’m running Windows 10. I also have legacy api password enabled and tried to login with my api password -> same error. So I tried it again in Chrome and there it still works. I also tried Microsoft Edge and there it is working as well, so I asume it’s a Firefox related issue. I tried to restart my PC, Home Assistant and even my Pi, nothing helped. I also tried to login on a second Windows device with Firefox and even there I get the same error.

Problem-relevant configuration.yaml entries and (fill out even if it seems unimportant): That’s my http config:

http:
  api_password: !secret hass_pw
  server_port: !secret hass_port
  base_url: !secret hass_ext_url
  use_x_forwarded_for: True
  trusted_proxies:
    - 127.0.0.1
  ip_ban_enabled: true
  login_attempts_threshold: 5

Traceback (if applicable): That’s what I get in the log file as soon as I try to login:

2018-10-16 18:07:51 ERROR (MainThread) [homeassistant.components.http.data_validator] Data does not match schema: required key not provided @ data['client_id']

Additional information: I’m using nginx to forward my hass instance to port 80, that’s the corresponding config snippet:

location / {
                proxy_pass http://localhost:8123;
                proxy_set_header X-Real-IP $remote_addr;
                proxy_set_header Host $host;
                proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
                proxy_http_version 1.1;
                proxy_set_header Upgrade $http_upgrade;
                proxy_set_header Connection $connection_upgrade;
 }