If you use mail.yahoo.com, you’ll see ads. The advertisement of Mozilla Firefox takes you to download version 17 of Firefox. However, I am opening the website with exactly that version of Firefox. Hence, such ad is useless. Yahoo’s web server should detect that I already have that version of Firefox. It can even be done in Javascript. Advertising Firefox 17 to users using Firefox 17 is simply inefficient.
Yahoo is down now ! The services is so bad :(