Yes they are free. Don't know why you added a Google search plugin, Firefox comes with it.

A place to get more search engines besides the ones Firefox comes with including Google is http://mycroftproject.com/

Check these out;

Add to Search Bar {web link} Make any pages' search functionality available in the Search Bar

Context Search {web link} Expands the context menu's 'Search for' item into a list of installed search engines, allowing you to choose the engine you want to use for each search.