Yahoo! points to the Firefox search box
I don't know how long they've been doing it, but if you visit the Yahoo! homepage using Firefox, after a short while a large banner ad loads at the top of the page, pointing out that you can search Yahoo! directly from the search box built into the browser.
1. Look for the search box in the upper right corner of your browser.
2. Click on the small triangle in the search box.
3. Choose Yahoo!. You're done!
I guess Yahoo! may be a little sensitive that Firefox ships with Google in the #1 position as default search plug-in. I couldn't make up my mind whether it was a very clever piece of marketing dressed up as targeted contextual help, or whether Yahoo! had just shouted "Hey, Idiot!" at me.