click on the favicon of the current search engin on search bar
scrool to bottom, click on ":Manage Engines"
grab the engine you wnat and move it to the top positiion, that is then your default search engine.
You probably were referring to changing the default at the Location Bar, but first
make sure you get rid of the Yahoo Toolbar, and keyword.URL is in the instructions,.
Yahoo Toolbar messes up Location Bar Search
Remove the Yahoo Toolbar
"Ctrl+Shift+A" (Add-ons Manager)
Select Puzzle piece on left-side for Extensions List
Find "Yahoo Toolbar" and click on "Uninstall" button, and restart Firefox. (if you really need to keep the Yahoo toolbar, you can hide the toolbar instead with customize, and still change all the search defaults as below)
Then to fix the default search engine used at the Location Bar, change the value of keyword.URL in about:config to one of the values shown in Location Bar search at MozillaZine.
type in about:config into the location bar (in a new tab)
if the warning message nonsense comes up uncheck the box before dismissing the dialog message
filter on keyword
right-click on keyword.URL and choose "modify" then paste in the Google search engine string of your choice.
filter on defaulturl
right-click on browser.search.defaulturl(if it exists)and again choose "modify" then paste in the Google search engine string of your choice.
filter on ytff
right-click on yahoo.ytff.search.popup_src(if it exists) and again choose "modify" then paste in the Google search engine string of your choice, if adventurous might see if "reset" would remove and see if gone in next session (well not for those keeping the extension).