I have an elderly friend who has Yahoo mail through ATT and there is always an advertisement at the top of his email list. I just told him to ignore it. Maybe I will try Ad Block Plus the next time I work on his computer.
If the person has Yahoo email through AT&T, you should be able to remove the ads through the control options.
I don't remember how I did it, but it is certainly possible to get rid of the ads if you're using AT&T.
Try using the link on this page
http://www.att.com/gen/privacy-policy?pid=24339where it says
Advertising on att.net: You can opt-out of receiving targeted online ads from Yahoo! when using our att.net email service or other Yahoo! services
***here***.
You must log in before that link will work.
Opting out of ads may not be an option for the free Yahoo email, but if you're using AT&T email you're paying for the email account. However it is "opt out" and by default you see the ads.
Seamonkey is a combination web browser/email client/newsgroup reader/html editor.
Think of it as the Firefox web browser combined with Thunderbird email client.
Seamonkey cannot do anything that Firefox and Thunderbird cannot do other than the html editing.
If the AdBlock Plus in Firefox doesn't remove an ad, the AdBlock Plus in Seamonkey won't either.
The main reasons to use Seamonkey are if you want an all-in-one web browser/email client, or if you prefer the interface, which is more "old-fashioned" by default (so you don't need to install an add-on like "Classic Theme Restorer" to get something you know how to use, the way you do with Firefox).