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.

You can check for problems with the localstore.rdf file if the above didn't help.

• http://kb.mozillazine.org/Corrupt_localstore.rdf

There is no plus sign (+) in the Customize window.

If I click the Restore Default Set, will it change the buttons within the Customize window only or will it also change the buttons in the toolbar?

Clicking "Restore Default Set" will remove all toolbar customizations that you've made and reset the toolbars to the default, so you will have to redo all your customizations.
If You do not see the plus sign on the Tab bar and there is also no New Tab button on another toolbar then your only option to get that new tab button back is to click that button.

I found the New Tab button (+) in the Customize window, which I could not find before, and followed your instructions to move it to the Tab Bar and to add a Flexible Space to the left of it. The Tab Bar now looks better and is more efficient.

Thank you for your assistance.