At the bottom of the list of search engines click "Manage Search Engines". In manage search engine dialog box, click "Restore Defaults", this should bring back all of the default search engines including Google.

You can also try to delete search.sqlite and search.json in the Firefox Profile Folder

If that doesn't work then you can try to reinstall the current Firefox version on top of your current installation or install the missing search engines from the mycroft site (http://mycroft.mozdev.org/)