It sounds like you have a third party program that has taken over your search engine, home page, and/or the default new tab page. Fortunately, this can be remedied easily:
After Firefox restarts, install the Search Reset Tool. This add-on will remove the rest of the traces of this program from your Firefox and then uninstalls itself.
For further information, please read Remove a toolbar that has taken over your Firefox search or home page.
Did this fix the problem? Please let us know!