What have you tried Alan? By 'Talk' do you mean type a message or something else.
Have you created an app with multiple chat rooms yet? Here is one approach How to create separate chat rooms
Show us your multi chat room Blocks and explain how the app should work. Someone might provide specific advice. It could be as simple as providing a List of open chat rooms and randomly selecting users to a room. Only you know how this should work so try some Blocks.
You might consult Programming Your App to Make Decisions to tell you app what to do when someone signs in. Be aware, all participants will have to have a copy of your app.
So far, no one has shown any code or tutorial for a FireBase version of the Omegle web based random chat (which probably uses a server). No one even knows whether such an app is even practical using FireBase.
Have fun with your Project.