Try the Firefox SafeMode to see how it works there.
A troubleshooting mode, which disables most Add-ons.
Troubleshoot Firefox issues using Firefox SafeMode

When in Safe Mode...

• The status of plug-ins is not affected.
• Custom preferences are not affected.
• All extensions are disabled.
• The default theme is used, without a persona.
• userChrome.css and userContent.css are ignored.
• The default toolbar layout is used.
• The JIT Javascript compiler is disabled.
• Hardware acceleration is disabled.

• You can open the Firefox 15.0+ SafeMode by holding the Shft key when you use the Firefox desktop or Start menu shortcut.
• Or use the Help menu item, click on Restart with Add-ons Disabled... while Firefox is running.

To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shft key) to open it again.

If it is good in the Firefox SafeMode, your problem is probably caused by an extension, and you need to figure out which one.
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+themes

When you figure out what is causing that, please let us know. It might help other user's who have that problem.

Thank you for the information! I found that the extension of "Ask Toolbar" was providing the interruption to the "open a new tab" function to the toolbar. After I disabled the "Ask Toolbar" function the "open a new tab" utility began to operate efficiently.

Thank you, the above helped. Even though the ASK toolbar was disabled, I had to remove it by going to the above menu and clicking successively upon:

• tools ->
• add-ons ->
• extensions (and then on the extension preceded by the "ASK" icon I clicked)
• "remove".

The new tab functionality came right back after restarting Firefox.