I think the sign-up page is broken in Firefox. I try to sign up and just get an error saying "We failed to validate the captcha response. Want to try again?", but there is no captcha on the sign-up page. There are various Google captcha JS pages being loaded, and there is a captcha logo at the bottom right, with links to terms and conditions, but no actual captcha dialog. This is when using Firefox 66.0.3.
When signing up in Chrome (version 73.0.3683.103) the sign up page works fine (but again, has no captcha).