I guess you installed it as an Add-on:
https://addons.mozilla.org/en-US/fir...o-for-firefox/
In which case click on "Tools" (at the top of your browser window), then "Add-ons" followed by "Extensions". Scroll down the list until you find it. Click on "Disable" or "Remove" (your choice) and restart Firefox.
If Google isn't restored as the default, click on the arrow on the left side of the search box and select "Manage Search Engines". Hopefully the rest should be self explanatory!
TFP