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.