This is a really cool concept - like really cool - but I feel that it's not being executed very effectively. Firstly, the little icon that appears in the 'email' fields of sign-ups doesn't allow you to click on the icon, select the email you want to fill it with, and then actually fill the email field. It wants you to re-direct to the actual page, tab over, copy it, then paste it into the email field. Horrible? No. Easy? No. Not too much, but steps can certainly be reduced for better user integration/convenience.
Secondly, MANY websites will not accept these 'mozmail.com' email addresses into their databases. Many will tell you that they aren't valid addresses. I'm not sure if this is an error on the user-end xp, or if it's an actual attempt to halt Firefox from being able to re-route marketing traffic. Either way, this is something that the dev team should certainly look into. I think this has awesome potential, but there are a few tweaks that could make this a mastapeace. (misspelled intentionally).

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It would appear that Firefox has not done their homework on the Relay addition. After I loaded it, I could NO longer enter my Google Gmail account as the email I had been using for years would not fully appear in the User field. They need to make the application user friendly by adding a simple ON and Off switch to which sites to be trusted and which sites to use Relay...DUH. What a huge let down Firefox! I'm not sure how to proceed at this point to retrieve my emails from Gmail. If you use Gmail or any other site where it automatically loads, DO NOT upload this software. You WILL have issues. I posted my issue on their site. Let's see if they will know how to fix it. Or, I will just try to find a way to delete it altogether.

I'm finding this to be a welcome addition to my information overload arsenal.

Some aspects of it are a little confusing (such as the reply functionality, which leaves me uncertain where my email response has actually gone), and I admit to being somewhat concerned by whether there might be privacy issues should the data held in 'Firefox Relay' be compromised.

So far, though, so good :)

Won't save labels for alias. They all revert to my email address. No way to use the custom domain I paid for either.

Couldn't get email from archive.org.

An extension from Mozilla for Firefox that does not work on Firefox. Sad, but unfortunately unable to use it even once, even after restart and similar trials.

It is impossible to join to Firefox Relay Premium from Ukraine - Stripe blocks payment either from card or PayPal. Ridiculous