This could be a malware problem or it may just be some plugin, such as a search toolbar, that you have installed that is affecting Firefox's behaviour. See

• searches are redirected to another site
• troubleshooting plugins - uninstalling
• is my firefox problem a result of malware

Thanks so much John99 this solved my problem. Scary going into the 'about:config' but I did and it is now working properly. My issue was only with the search toolbar.

Glad to have helped.

For anyone else having problems I would also add that there are articles about Firefox's Search features that may be of interest:

• Location bar search
• search bar