Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions 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.com/kb/Safe+Mode

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

In Firefox 4 you can use one of these to start in Safe mode:

• Help > Restart with Add-ons Disabled
• Hold down the Shift key while double clicking the Firefox desktop shortcut (Windows)

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

The Ask plugin/toolbar is a regular troublemaker. Google for information and advice.

It would help us and other users to hear just what the problem was and how you fixed it.

Thank you. I went to "View" and when I turned the ASK toolbar back on I found in the options of the ASK toolbar an uninstall option but it did not get rid of the tab display. The tab display is now gone since I disabled the add ons in Safe mode. Thank you.

I went to simply control panel - add/remove and removed/uninstalled whole thing. Listed there something like "Ask toolbar".

Before trying cor-els solution I checked the list of installed programs in control panel and found out that in my case installing KMPlayer had caused Ask.com toolbar to appear in firefox when opening a new tab.

Removing/uninstalling it and restarting firefox solved the problem for me.