Well, guys, I think I've discovered the answer to this. It has nothing to do with what browser you're using. I found
http://info.yahoo.com/privacy/us/yahoo/relevantads.html. There, if I click on the button titled
Manage, that takes me to a page where I can select categories of ads I prefer seeing or completely opt-out of all ads. If I elect the latter, I get an ad-free mail screen, just as you do.
So, apparently, you've opted out at some point. I believe the opt-out can be put into effect when first creating a new Yahoo! email account based on choices you make during your initial registration. So, if you don't recall ever opting out after creating your email account, you probably made choices when creating your account which resulted in opting out of all ads.