First I would try this: http://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problemsThe other thing you could do is go in to about:config, search for the name of the toolbar. There you should find some stuff, set them to deafult.If you only want to set back this new tab thingy, you should search(in about:config) for browser.newtab.url, and set that to about:newtab.
where is about:config?
You have to write it in firefox's address bar(where you usually write web addresses).