Oh, you just need to update to the latest version of Firefox (3.6.11). Your version is out of date and not secure. http://www.mozilla.org/security/known-vulnerabilities/firefo...
On 802.11 wireless networks your wireless network card is capable of capturing traffic addressed to other computers. When encryption isn't used or is compromised, you can steal their credentials.
Doing something similar against cellular networks would require a much more sophisticated attack with specialized hardware that's largely illegal in the United States. I would also hope that cellular communications are encrypted these days.