Hi,

After updating Jenkins from 2.420 to 2.429 (and above ) pop-up menus in Jenkins became transparent when using Firefox browser, could you please fix that?

Steps to reproduce:
1. Open/Create a pipeline
2. Enable "This project is parameterized" in General section, or "Use secret text(s) or file(s)" in "
Build Environment"
3. Press Add button to add a parameter/secret
 
Expected results:
Menu with clear items selection is shown

Actual results:
Transparent menu is shown, where items are overlaying the background.
 
Additional info: tooltips seems affected too, see the 3rd screenshot.
 
In Chrome-based browsers menu renders correctly with some fancy transparency to the edges (Chrome-only styling?)
css class for those items is .jenkins-dropdown, looking into source code I see this commit, which may be related
https://github.com/jenkinsci/jenkins/commit/0c71e4105cb5d5301d45b42a16f511c6df30034c