Start Firefox in Use Troubleshoot Mode in Firefox to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox (Tools) > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

The problem stemmed from confusion associated with View -> Toolbars -> Customize. I did not want the "New Window" button on my toolbar, which brings up a second FF browser. There is an "Open A New Tab" button which can be placed on the toolbar, that's the one I was missing. Oddly, when you go into "Cutomize," a plus sign which looks alot like the "Open A New Tab" button will appear at the top right on the toolbar.

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.

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