When you go to yahoo home page using firefox browser, you will see a box at the top of the page with text that reads Search with yahoo! right from your browser. Then another column, it says: 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!
Now, funny, I noticed after the last firefox update the small triangle in the search box has gone!