There is a rather fiddly way to do it:
1. In the Location bar, type about:config and press Enter. The about:config "This might void your warranty!" warning page may appear. Click I'll be careful, I promise!, to continue to the about:config page.
2. Type browser.newtab.url in the search box. Double-click the browser.newtab.url preference and change the url from about:newtab to about:blank. Alternately, you can change it to about:home for the Firefox Google home page, or type in your preferred home page, for example google.com.
3. Click OK and close the about:config tab.