Thanks for checking out TogetherJS.

You should see a link/button at the bottom right of the browser: togetherjs — if you don't then you need to show the Add-on Bar:

Go to View > Toolbars > Add-on bar to display the bar (or read more here).

Once you see the button, click it to start sharing your session. Once you are sharing you have to tell someone else (more instructions here)