That will be due to an add-on. Check for any Yahoo add-ons to see if any of them have added this. Another add-on that does this is the freeze net assistant, if you have that one you can uninstall it and Firefox should revert to showing a blank tab.

There are add-ons that let you specify what to display on a new tab, such as the NewTabURL add-on, but unless you find out what add-on is causing it to open with yahoo and disable/uninstall it, it is best not to install NewTabURL as it may conflict with the other add-on.

Thank you Tony! I had already looked for Yahoo add-ons, but I had no idea what Net Assistant was, and it was indeed the culprit. I removed it from my programs via the control panel, and voila! The Yahoo search page is now gone from my new tab page.

It seems "net assistant" and other yahoo stuff, such as yahoo toolbar, were added the last time I upgraded Firefox. It's a little disappointing that Firefox is sneaking these things into its upgrades now.

You do not get those when updating Firefox, they can be installed when you add another program/utility, some have ridealongs such as toolbars for IE and Firefox.