Ahh, I got it, this specific page works okay:
`https://account.protonmailrmez3lotccipshtkleegetolb73fuirgj7r4o4vfu7ozyd.onion/start`
This one attempts to send clearnet requests:
`https://account.protonmailrmez3lotccipshtkleegetolb73fuirgj7r4o4vfu7ozyd.onion/mail/signup`
