Disable or uninstall the Ask toolbar extension. It has a known compatibility issue with Firefox.

I had done so over a week ago, and also disabled some other things (like the annoying Skype highlighting).I wonder whether what I did then prompted this problem with the tabs. Do you have any other suggestions? Thanks!

Do you get the same result if you double click the Tab Bar itself, to the right of the plus (+) sign?

Nothing happened when I did this.

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

It worked..I thought I had done so already but turns out I had only removed it from the bar below the address. Thanks!