You have to tell FF that you want a new tab to open with your homepage url or some other webpage.
In the Address bar type, about:config and press Enter.
The about:config 'This might void your warranty!' or 'Here be dragrons' warning page may appear. Click 'I'll be careful, I promise!', to continue to the about:config page.
Type, browser.newtab.url in the search box.
Double-click the browser.newtab.url preference and change the string value url to what you want, you can change it to about:home for the Firefox Google home page, or type in your preferred home page, for example google.com. (you don't type the www's)
Click OK and close the about:config tab
Now when you open a new tab that is the page it will display.