Accessing the internet
Error message says "you must log into this network before you can access the internet. After I click on another button, message says "success. https://detectportal.firefox.com/success.txt
This is the result of a security feature your network administrator has implemented, requiring everyone to log in before using their network. Any page you attempt to load before logging in will attempt to redirect you to the login page. This does not work if you are trying to connect with HTTPS, as its security does not allow incoming data to be intercepted. (If you want a page that never uses HTTPS, go to neverssl.com.) Some browsers or operating systems will automatically tell you that the network requires login and offer you to visit the login page automatically. After the login attempt, as a Firefox user, you are redirected to the Detectportal website you mentioned so that you can verify that you are online. If the success.txt document loads (its content is the word success) then you are ready to use the internet. If you want to stay logged in permanently or change something about the login procedure, contact your network admin.
If you do not need to use this service then you can set network.captive-portal-service.enabled = false on the about:config page.
You can open the about:config page via the location/address bar. You can click the button to "Accept the Risk and Continue".
See also "Network Detection".