We don't currently support sending messages with relay aliases. Sorry.

Sorry to hear that. Until we can fix it, you can disable the relay button in the settings panel. You will still be able to generate relay aliases by right-clicking in an email field.

Howdy. You should be able to delete relay aliases on your account profile at https://relay.firefox.com/accounts/profile/. Click "Show details" on an alias to get to the Delete button. Deleting is irreversible and can potentially lock people out of accounts so we don't want anyone to accidentally delete an alias.