Based on the extensions that you have installed on Firefox, my assumption is that this behaviour is caused by either the Norton Safe Search or Search Secure Pro extension.

I'd recommend trying Firefox in Safe Mode to see if the issue goes away, since Firefox's Safe Mode option will temporarily disable any extensions that you have installed on Firefox.

I'd also check to make sure that you have the correct default search engine selected in Firefox's search options.

Hope this helps.

See also:

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware

My search box suddenly says "enhanced by Google" and I'm getting ads at the top of my searches

In addition to rooting out any bad add-ons or other malware, if this unwanted search page shows up in every new window -- you can test using Ctrl+n -- check your home page setting. More info on that in this article: How to set the home page.