You can change the search engine to Google in the Search bar - add, change and manage search engines on Firefox using the drop-down menu, as shown in the article here: Switching search engines

Sorry-I must have used the wrong term. when I said "Search Bar". I didn't mean that bar, but the bar on the left side of the page. Perhaps that's called the Address Bar? I usually just use that instead of the Search Bar---I just put whatever I'm looking for in there, even if it's not an address, and it would default to Google and search for me. Now, it defaults to Yahoo for the search.

The Firefox "address bar" is known as the "Location bar". The article that describes your issue is Location bar search - it explains how to change the Internet Keyword search service in the Internet Keyword search section, under Changing the keyword service

To change it to Google (the default service), enter about:config in the Location bar (address bar), find the preference keyword.URL and then right-click on it and select "Reset", as explained in the article.

P.S. I'm wondering how it got switched to Yahoo in the first place .... maybe a Yahoo toolbar or other add-on you installed?