(Relay engineer here.)

While we provide a Firefox extension with which generating an alias is just a click away, you're not dependent on Firefox specifically: you can generate and access your generated alias through the web interface at https://relay.firefox.com in any browser.

A little bit unrelated question, but since we're talking about being available anywhere. Is there a reason the extension is not available for Firefox on Android? I'm thinking about using Relay but I'm afraid it won't be easy to add aliases from mobile.

No technical one I think; probably mostly a matter of resources (same reason it's not on other browsers yet).

That said, the extension is mostly useful for generating new aliases. If you've already used it on desktop to generate an alias for a website, then regular cross-browser sync of form autofill data should make it easy to reuse the same alias on mobile.

Correct me if I'm wrong, but you would still need to pay $3 USD/mo for Fastmail even if you use 1P. Whereas with Relay, it's 0.99 USD/mo, and no need to migrate my existing email to any other service.

Well yes, it is much better if you already use Fastmail.

Right with you on Fastmail, it's excellent. Just wondering though where do you feel you can't use Firefox? As far as I know it runs on all major platforms even if the rendering engine on iOS is still Safari.

I find on iOS it just isn't as well integrated as using Safari.

Also, apps.

Edit: I am trying it again and it seems a lot better integrated than when I last tried - you can set it as your default browser! Is there a reliable way to block ads, though?

Sign In with Apple is a regular OAuth service and works fine in a browser.

You can use Apple’s solution even if on Linux and using any browser.