Using the link given in the avast alert, I tried that and it redirected to hXXps://support.mozilla.org/en-US/kb/captive-portal without issue (see attached image). Note I have intentionally broken the link so it isn't active.
I have no idea why Firefox (firefox.exe, the executable responsible for the connection) would go there, but my manual attempt got through without issue. That said see the lower highlighted text in my screenshot, it could have been a legitimate connection.
Not I'm also not using Avast One, but Avast Antivirus Free, but I doubt that should differ as far as detections are concerned (other than a different Alert Screen).
