QuoteIf you don't have full-session HTTPS turned on for your webmail, anybody on your WiFi network could read any of the emails you write and receive using a tool like FireSheep, as they are transmitted from Yahoo to your browser.
That's because, without HTTPS, they are sent as unencrypted text.
If you don't know how to turn it on, see the article at Using Yahoo Mail? You should turn on this privacy option as soon as possible | Naked Security (http://nakedsecurity.sophos.com/2013/01/08/yahoo-mail-https-ssl/)