Hello,

Assuming that this isn't a simple need to set your search engine, the instructions to remove a search hijacker are below.

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 The image "fx57menu" does not exist. 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.

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!

Just a note, no Firefox version for any locale since the Fx 57.0 Release has come with Yahoo as one of the search engines. And before anybody claims Yahoo search engine is still in Firefox see https://support.mozilla.org/en-US/forums/contributors/712925

So I agree this could be from an Extension you installed, a software you installed or updated recently, a malware or something else.