Hi Rory, do you mean on the built-in Firefox Home / new tab page? You can hide the search bar, or you can change it to a different provider. The following articles should help with that:

• Settings page > Home panel, and/or: Customize items on your Firefox New Tab page
• Settings page > Home panel, and/or: Manage Firefox search settings

Or if you are referring to the search bar on the toolbar. https://support.mozilla.org/en-US/kb/add-search-bar-firefox-toolbar

If you are seeing Google then that means you have Google set as the default search engine.