Something has changed the default settings, it may be some addon or plugin, or even malware. See Troubleshoot Firefox issues caused by malware

There are various posts about smilar problems and there will be suggestions on sites or software to use for malware scans.

If it merely needs changing to use google See thread

This problem is the result of a Trojan named Supsav.Smss32. I decided to download Spybot Search and Destroy to help me detect the problem. This trojan would not let me access the Spybot web page. I used another computer and went to www.spybot.com and downloaded the Search and Destroy program. Spybot was tricky to install because it wanted to access its home page during installation, something the Trojan would not let it do, therefore the installation would stop. I unchecked the box that allowed for immediate updates and Spybot installed. I immunized my system with the protections offered in the download version of Spybot and ran Search and Destroy. I was amazed with what Spybot found. I removed all the Spybot found problems but still could not update Spybot from the Web. My McAfee product had not been doing a very good job. I had to run Spybot several times before it identified and removed Supsav.Smss32. After the trojan was removed, I could use the Spybot update feature and update Spybot. Note: When you download Spybot to a flash drive you can also download a definitions update. I downloaded both and installed both. Sorry for the long answer but this problem has been bothering me for several months. It is now gone. I also made a voluntary contribution to Spybot - something not required. This is an amazing free product. Good luck!