I use Firefox Relay but I'm super unhappy with it. They only offer 1 domain (or subdomains for it in paid version) and this domain is blocked on many websites.

I use Firefox Relay but I'm super unhappy with it. They only offer 1 domain (or subdomains for it in paid version) and this domain is blocked on many websites. You can't send mail from a relay alias. Too few aliases available in free version.