You can find the New Tab button showing as a '+' on the Tab bar.
You can open the Customize window and drag the New Tab button that shows as a plus sign (+) from the Tab bar on another toolbar and it will become a regular toolbar button like the New Tab button that you have in Firefox 3 versions.
If you can't find the New Tab button then click the "Restore Default Set" button in the Customize window.
If you want the New Tab button at the far right end of the Tab bar then place a flexible space to the left of it.

See also:

• New tab toolbar button: https://addons.mozilla.org/firefox/addon/new-tab-toolbar-button/

With limited success, I'm still a little disappointed. Why isn't there a new Tab icon in the Customize folder? I've been able to restore the + however, it's so small.. I also installed the "new toolbar button" and restarted, but I see no difference... I wish FF would restore the New Tab toolbar icon..

You can drag the New Tab plus symbol that you find on the Tab bar on another toolbar and it will become the regular New Tab toolbar button as present in older Firefox versions as I posted above.

Open the Customize window via "View > Toolbars > Customize" or via "Firefox > Options > Toolbar Layout"

• http://kb.mozillazine.org/Toolbar_customization
• https://support.mozilla.com/kb/Back+and+forward+or+other+toolbar+items+are+missing