If your video stream isn't showing video from your webcam, there's a chance you haven't given permission for LearnCube to access it.
Solution: Allow your camera
Test your equipment by going to https://app.learncube.com/troubleshooter. You'll get to the webcam tests, remember to select "Remember this decision" (so that you don't have to approve it every time you enter a classroom) and click on "Allow".
When entering a class, click on the site information tab and you'll see the permissions you've given to LearnCube to access Firefox:
If you're ever entering a class and you see a prompt to allow the camera, please make sure you click Allow to start it.
Now you should be all set. Enjoy your class!