First issue encountered today. Tried to change my email with Craigslist but was unable to.
Craigslist required confirmation from old and new email addresses. Old email address received the confirmation request and this was confirmed.
However, Firefox relay forwarded (as it is supposed to) the new "relay" email. So when I tried to confirm it, it only showed the old email to Craigslist.
I wonder if this issue will occur with other site that I want to change my email to the "relay" email?
See below:

We have received a request to change the email address associated with your craigslist account.

If you did not request this change, please log in and change your password.

Old address:---------hotmail.com
New address:--------relay.firefox.com

You will need to confirm this change from both the old address and the new address.

You have 60 minutes to submit both confirmations.

Visit the link below to confirm this change: