Hi chickennugget, is this on Android? Usually, there's a top bar notification to load a sign-in page so you can do that before any apps need to connect.

As a more general approach for any OS:

When a wi-fi router replaces your request for an HTTPS page with its sign in page, it is normal to get an error that the domain doesn't match the certificate.

Workaround:

First load an insecure HTTP page such as

http://example.com

so that Firefox doesn't block access to the sign-in form. Then you can browse normally after that.

no, its on PC.

Okay, next time, try connecting to an HTTP (insecure) page to get through the authentication wall and see whether that works for you.

it still won't work. It said it is connected, but usually i have to sign in on this network to be able to use the wifi. the problem is, the sign in page won't load. it said either "connection is not secure" or "Hmm. we're having trouble finding that site."

Did you get a chance to test it already?

If a new HTTP (insecure) page doesn't work in Firefox, try it in Internet Explorer. Same principle -- the browser doesn't like secure addresses to be hijacked by authentication pages, but it's fine with insecure pages being hijacked.