Go to View, Toolbars, Customize. You can set up the New Tab button there. Also go to Help, Using Mozilla Firefox, Navigating Webpages, Tabbed Browsing to learn how to use the Tabbed Browsing.

slimbrowser is INternet Explorer, just with a different look
No, slimbrowser is NOT Internet Explorer. Internet Explorer is the standard browser created by Microsoft and included with Windows. Slimbrowser is a browser created by Flashpeak which is compatible with IE and most, but not all IE plug-ins. You must however, have Internet Explorer installed on the computer for Slimbrowser to work, but it is NOT Internet Explorer.