I am using gmail and wanted to create a new key pair for sending emails.
I can receive the email to verify the key. After entering the password I receive a "User id not found" in the Firefox.
After that I tested the same procedure with Chromium. It works there.
So that isn't any bug of the operating system.
Link to "User id not found": https://keys.mailvelope.com/api/v1/key?op=verify&keyId=e060293bba13aefb&nonce=5c42fe37d73d48b2ae6f82f9bc457515
Firefox version: 60.2.2esr (64-Bit)
Operating System: openSUSE Leap 15.0