I have run into many places that will not accept my FireFox email addresses.

The add-on shows already deleted addresses.

Easy to set up, about as clear as mud to use.

Features are nice, but I have multiple email addresses, and only being to setup forwarding to only one really sucks. I really like how there's an option to label generated addresses.

Trochu nanic

While the extensions is a fantastic resource it can interfere with the proper functioning of some pages. As an example, I can not log in to the Wolfram|Alpha website if the extension is enabled

idea behind the service is great, in real world, relay icon almost always ruins the login form in terms of usability or design, sometimes completely hiding email im typing in. inability to copy mask from a relay menu in toolbar kinda ruins whole plugin point? also why im forced to read news every time i click on the plugin icon?

Clicking the addon button in browser brings up an Ad page for Relay, VPN, etc. To even see your Relay email, you have to click "Manage all masks" just to bring up a link to relay site on another tab. You can't even copy and paste from the Add on button because your mozmail emails are not even displayed.

Works fine for me and serves it's purpose. Lots of people down-voting or crying about it but probably still use FB. The process is simple, most of the problems are user specific or can't read instructions. The only thing I don't like is the addon button does nothing, doesn't even show you your mask, just a giant ad in the middle. You still have to click away to another site to copy and paste. The relay auto detect for field is also hit or miss. As far as getting emails relayed, works well and I haven't had issues

There isn't a critical mass of users, so websites are already starting to block this domain. iCloud on the other hand has the critical mass and no one will dare to block their domain.
This should be a feature of an email provider. Although Mozilla attempting to be a privacy provider, these products are half-baked and quite a commitment on their part while the project may not last.