I"m guessing that what happened is that you or someone inadvertently allowed the default search method to be changed in the keyword url search.

Search for any add ons or unwanted plugins and remove them. Then if that doesn't have the effect that you want, go to your url window and type in about:config.

Its going to tell you that you are voiding your free and not often used warranty.. Just click the "i'll be careful I promise button.

you will see a full list of things in alphabetical order. Scroll past them and find Keyword Url.

Right click on that and select modify.. type in google..Modify the search string to read google and then close your firefox and reopen it. Your default google search should work when you type in the keyword terms you're looking for.

https://support.mozilla.com/en-US/kb/Search%20bar

See - Managing search engines