I love it. It works as expected. 5 alias limit might seem too restrictive, but at the moment I am not impacted by it.
I miss two things
1. my own alias which many have reported
2. Change my forwarding email address. Right now it only takes email from firefox account.

Nov 1st, 2020;
Without the ability to reply using the Firefox relay alias, the very purpose to use Relay is moot!
I contacted a car dealership using one of my aliases. However, I had to reply using my own personal email, which got captured by the dealership, and I am now permanently entered in his distribution list.
So, what is the point?
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I would like to have the option to create my own prefix email, using the @relay.firefox.com. Without the limitation of 5 aliases.
In this manner, I can control which email is being spammed, example:
I create an account with a new entity; I create the specific alias:
-Mynametag.newentity@relay.firefox.com
If I start to receive junk mail from "newentity", then I delete the alias.
I have done that for many years, unfortunately, it means that I need to create a new email address each time I enroll, which means my computer currently reads tens of email addresses.
So you should give the option to the user to create his own alias, and remove the limit of 5.

Edit on July 31st, 2020:

To avoid collisions, or similar prefixes, you can always have a validation running at creation time; I also add a string to make my alias unique, such as the date at which I create it:
20200731NewEntity@p6.firefox.com
This has two benefits: I can organize my email aliases list in chronological order, increasing or decreasing, and I always keep track of the date at which I created the alias.

If a user has many Relay IDs and happens to get emails which for some reason are not easily identifiable from the content as to which account sent them, then it would be useful if the information in the forwarding header also included the information from the Account note field which was provided when the ID was created.

i currently have burner email extension which allows you to use 5 email addresses which can be used over and over again without logging in. i was expecting relay to be the same but it seems i have to log in to use it every time, and when i close the tab or firefox browser, i have to log in again.i'm guessing i'm missing something : ) or not ? but either way i'm sticking around to watch it grow , thanks its still useful though, just a bit different.

sorry for late response, after using a clean profile i'm guessing my problem was an addon clash as it works as expected now : ) gonna check them out by disabling one by one - thanks

so it is temporary containers addon that was causing it for me.

So far, so good.
Work as expected.
Easy setup.

Seems to work very well. 4 instead of 5 stars because only 5 aliases is not enough for my needs...but I guess this will maybe increase in the future.

Experiencing the same issues as a lot of other folks, I joined the waitlist over a month ago, finally got an invite email to install today and start using it, yet when I follow that link it prompts me to sign in and then asks me to join the waitlist... the one that I was already on. Makes no sense, very frustrating. Worth mentioning I'm in the USA, since it's only available here right now for beta. What gives, Firefox? :(

Edit: I figured it out so I figured I would update my review. Make sure you are logging in with the email address they sent the invite to. Apparently I use different emails for Firefox overall, and what I signed up for Relay waitlist. Don't even remember having a Firefox account for that email but, there you go. Hence the confusion. I think maybe the email could just give you a friendly reminder to do this, to avoid user error like mine (and maybe a few other folks).

once installed and that purple icon appears inside email login field its capturing the tab key so cannot simply type in emailid hit tab and enter password ... that purple widget should NOT capture the tab key when cursor is inside login(emailid) field ... Please fix

This extension achieves all that it says, and works perfectly; from detecting signup pages to syncing with the dashboard (as well as the ability to label accounts).

There is however a very small issue in terms of ease of use, that would be auto filling previously used alias' from the popup menu created by the extension. There is only an option to "Create a new alias", and adding a list of previously created alias' would be helpful. This is a minor issue, and not something that affects the performance of the service itself.

In addition, another feature for the future could be the possibility of listing all the websites the email is used on.

Keep up the great work! I'm always hyped when Mozilla releases a new service because it never disappoints.