Try Ctrl+t

Most of the shortcuts should be listed on the full menus. To display the classic menu bar, tap the Alt key or press F10.

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Oops, sorry, Ctrl+k duplicates the current tab. To do that in Firefox, click in the URL of the page and press Alt+Enter.

There also might be an add-on that creates a more convenient method.

You can middle-click the combined Reload/Stop/Go button on the right end of the location bar to duplicate the current tab to a new tab.
You can also press the Ctrl key and drag the tab left or right on the tab bar to a new position to copy a tab.

The only way to do that with the keyboard is to use Alt+Enter like jscher2000 wrote above. That only opens the current URL in a new tab.

Thanks cor-el and jscher2000 both of these worked I really appreciate the feedback. I was having a problem with my Windows 7 computer and my IT guy re-installed the os, etc. When he did he installed ie9. I was having a huge problem with ie9 and he tried to re-install ie8 but Windows 7 would not let him. Since I need the internet he installed Firefox which seems to work well.