It could use the icon still included in Firefox. Furthermore, when used with New Tab Homepage View, the Google search textbox doesn't get focus.
There's still a new tab button functionality in Firefox, until you remove it from your toolbar. That is it's not visible in the Customize Toolbar dialog, but it's still there.
To add it manually, open "Help>Troubleshooting Information", then click "Open Containing Folder" and open the localstore.rdf file in your profile.
Then look for the second occurence of "chrome://browser/content/browser.xul#nav-bar" and paste "new-tab-button," (with the ",") after 'currentset="' (after the ").