They've got it plastered around the UI. It's the very top menu item. It's also in the Pocket address bar button right under where they advertise to you to "Sign up for Pocket. It's freeeeeee!"
Not only that, but on new installs and after some updates Firefox nags you to sign up whenever it can, for instance when you log into a website - right after you save the password it will show some animated crap in the address bar to nag you to "Sign in to sync..."
I just opened Firefox (on Linux no less) to confirm every single one of these things.
Chrome (my default browser) actually nags me less and in less annoying ways.
