That's mysterious. Here is how you can reset what the tab does. If this setting does not stick, one of your add-ons might be overriding it.

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the filter box, type or paste newtab and pause while the list is filtered

(3) Double-click the browser.newtab.url preference and enter the desired value:

(i) Page thumbnails (default)

about:newtab

(ii) Blank tab

about:blank

(iii) Built-in Firefox home page

about:home

(iv) Any other page

full URL to the page you want to appear

Press Ctrl+t to open a new tab and verify that it worked. Fixed?

Thank you so much. Ive been trying to fix this for days and with your solution it took seconds.

You're welcome. And apologies for giving you that Windows keyboard shortcut instead of command+t for Mac users.

I have tried this. I got rid of the unwanted url but when I put in about:newtab to get the new tab page which has the icons of the sites I've pinned there, I get a blank tab instead. What do I put in there to get the icon page back?

I'm on Windows 7. Firefox 24.

Hi samintoronto, does the page have a little grid icon on the right side to show and hide the thumbnail, or is it completely empty?

Extensions can override your setting. Disable ALL nonessential or unrecognized extensions here:

orange Firefox button (or Tools menu) > Add-ons > Extensions category

Usually a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Any luck?