Can't login with Firefox browser

So I can't login while using the Firefox browser. When I enter my name/email and passwort and press the Login button, I'm being directed to the frontpage and a message appears that says "logged in" but I'm actually not logged in. Top Right it still says "login" and "register"and I can't access my profile.

The problem occurs on Desktop and Android version of Firefox. I have no problem loggin in while using Chrome or Edge browser.