Switch camera/microphone input on Firefox

To switch your camera and mic on Firefox, click on the video/mic icon on the left of the address bar:

Then, in the popup, press the “x” to cancel any existing mic/camera permissions:

When you reload the browser, Firefox will ask for the permissions again. You can then select your preferred camera/mic input source using the drop-down menus. If you choose, you can also remember this decision so you don't get asked again: