I've had a YaHoo account since earlier this year.  (Free email account, anyway.)
I don't use it for much, but I do drop in via Mozilla.  I don't remember
what browser I used to set up the account; maybe Mozilla, maybe Netscape
4.x.  (shrug)

At the risk of insulting your intelligence, have you tried the usual suspects?
(Cookies & Java.)

They might also be doing something really dumb about browser identifications.
Try different browsers, maybe?


-- 
  "I probably don't know what I'm talking about."  http://www.olib.org/~rkr/