[Bug]: First login attempt with Mozilla Firefox (on MacOS) is ignored

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

When I open a fresh session with a Mozilla Firefox browser and try to login to Nextcloud the first attempt is completely ignored. The page reloads and when I enter the credentials again, everything works fine. I have noticed this behaviour on two separate Nextcloud instances, we operate at our University. Both are on the latest version (with German language pack).

Unfortunately, I'm not the admin of neither of these instances, but I tried to find a solution with one of the admins who told me that he doesn't even see the login attempt in the logs.

With other browsers (such as Chrome) it seems to work fine. So no idea whether this is a bug in Nextcloud or Firefox, but I decided to start here. ;-)

Steps to reproduce

1. Take a Mozialla Firefox (122.0 in my case) on MacOS (14.2.1) and start a fresh session with a Nextcloud server.
2. Enter credentials on login screen. Press the "Log in" button.
3. Page seems to load, but it reloads the login screen, no matter whether the credentials were correct or not.
4. The second login attempt works as expected.
5. Trying this after e.g. a successful logout or when there was a recent session with the Nextcloud server it also works fine.

Expected behavior

I would expect to either get logged in after step 2 or get a bad credentials message.

Installation method

None

Nextcloud Server version

26

Operating system

None

PHP engine version

None

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

Start by reproducing the fresh-session login flow described for Firefox 122 on macOS 14.2.1 against Nextcloud Server 26, checking whether the first attempt reaches the server logs. Done means the first submission either authenticates successfully or shows a bad-credentials message instead of silently reloading the login screen.