I have installed second USB camera and it works for me too.
Please check your media settings in Firefox and check if camera is blocked.
You can also check these links to check your webcam with FF: https://webrtc.github.io/samples/src/content/getusermedia/gum/ https://appr.tc/ https://test.webrtc.org/
If all of these links work for you, please provide your Firefox version and OS version.
We will try to check it on your OS.