From the Mozilla web site. It's difficult to find, though.

Mozilla has a Firefox Hello Terms of Service:

https://www.mozilla.org/en-US/about/legal/terms/firefox-hell...

which claims to link to the Firefox Hello privacy terms, but actually links to the main Mozilla privacy page:

https://www.mozilla.org/en-US/privacy/

which, in a sidebar, links to the Firefox Hello privacy policy:

https://www.mozilla.org/en-US/privacy/firefox-hello/

which links to the TokBox privacy policy:

https://tokbox.com/support/privacy-policy

which says:

"We use the information we collect from you in the following ways: ... To organize and carry out TokBox’s marketing or promotional operations/offers, contests, games and similar events."

One of Hello's basic functions, at least in its mobile forms, is to cross-reference your phone contacts and Facebook contacts. See "Review: “Hello” Facebook Dialer, Bye to Your Privacy?"[1]

So, yes, it snoops on your contact information.

[1] http://www.xda-developers.com/review-hello-facebook-dialer-b...