Assuming you're running Firefox esr 102 rather than running Firefox 112, this is a known issue with that version of Firefox. In the duplicate report there is a comment that says:
I see this bug on Firefox 102.9.0esr, when using Credentials Plugin Version 1224.vc23ca_a_9a_2cb_0 on Jenkins 2.387.1.
I don't see this bug with Firefox Developer Edition 112.0b9 (64-bit), nor with Microsoft Edge Version 111.0.1661.62 (Official build). In those Web browsers, the add credentials button has no visible graphic in it.
If you're not running Firefox esr 102, please provide the version of Firefox that you are running.
Assuming you're running Firefox esr 102 rather than running Firefox 112, this is a known issue with that version of Firefox. In the duplicate report there is a comment that says:
If you're not running Firefox esr 102, please provide the version of Firefox that you are running.