Some users have trouble signing in on WordPress using the Mozilla Firefox browser. They get the message: “Oops! That page can’t be found.” Or the login just doesn’t work.

A possible solution is a change in the security settings. Open the about:config page in Firefox and check the settings for:
network.http.sendRefererHeader

If the value is 0 the login doesn’t work. Set this value to 1 or higher and the login works again.