Please don't assign individuals to issues, unless the individual explicitly stated they are willing to contribute to an issue. 

I'm not interested in contributing to issues occurring in specific plugins for Firefox ESR versions only.

I have unassigned myself.

However, you are very welcome to contribute a fix towards the credentials plugin

Sorry. I thought you would be interested since you were assigned to last similar one. My bad and not gonna happen again.

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. Apparently, the plus sign exists there but it is scaled to zero size (or almost that) due to the browser style flex-shrink: 1.

Because the plus sign does not show up in those two browsers, and other dropdown buttons such the "Add" button under "Additional Behaviours" in pipeline configuration do not have any graphics, I think it would be OK to delete the plus sign from the add credentials button. I mean delete <l:icon src="symbol-add"/> from src/main/resources/lib/credentials/select.jelly#L120 and perhaps also from src/main/resources/lib/credentials/select.jelly#L152.

> I am using FireFox, I see that might be relevant from past similar issues.

The plus is giant on firefox, but slightly oversized on other browsers too.

I recommend switching away from YUI elements to APIs provided by core, which is no longer reliant on YUI for buttons, to resolve this incident and prevent future ones.