How do I allow Firefox to have access to camarea?

Firefox will not allow the access.

If a site need your camera, there will be a notification asking for the permission explicitly.

This is how the permissions are granted during browsing: How to manage your camera and microphone permissions with Firefox

On first use, if not blocked previously, Firefox will request the access to the devices from your system. If you didn't allow that, you'd need to consult your System Settings, and manually look up Firefox in there and see if you need to grant it device access from the macOS side of things: