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:

1. Click the menu button and choose Add-ons.
2. In the Add-ons Manager tab, select the Extensions panel.
3. Select the toolbar you wish to remove.
4. Click the Remove button.
5. Click "Restart now" if it pops up. Your tabs will be saved and restored after the restart.

After Firefox restarts, install the Search Reset Tool. This will remove the rest of the traces of this program from your Firefox.

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!

This is your current setting;

keyword.URL: https://au.search.yahoo.com/search?fr=greentree_ff1&ei=utf-8&ilc=12&type=275451&p=

Check your programs folder, and remove Yahoo anything.

Windows: C:\Program Files C:\Program Files (x86) Mac: Open the "Applications" folder. Linux: Check your user manual.