Hmm, normally the new tab page is controlled by Firefox unless an extension overrides it. Could you check your extensions on the Add-ons page. Either:

• Ctrl+Shift+a (Mac: Command+Shift+a)
• "3-bar" menu button (or Tools menu) > Add-ons
• type or paste about:addons in the address bar and press Enter/Return

In the left column of the Add-ons page, click Extensions. Then cast a critical eye over the list on the right side. Any system extensions that Firefox installs on its own are hidden from this page, so everything listed here is your choice (and your responsibility) to manage. Anything here mention a new tab or anything associated with your home page? If in doubt, disable.

Can you get back control?

Fantastic! Thank you so much jscher2000! The add-on extensions stated that Yahoo's homepage would pop up as my homepage and as my new tab page. Now, after deleting the extension, new tab pages now appear with the tiles showing most visited sites. And Yahoo's home page pops up only when I click on the "house" icon--which I want (and I will assume it will appear as my homepage when I log on, too; if not, I will just click on the house again, or try to add it as a homepage, only).