It don't? That's sad! Works fine for me.

Sorry for my english, i am from argentina. I try reinstalling but still don't work..

Try to set the pref browser.tabs.drawInTitlebar to false on the about:config page.

To open the about:config page, type about:config in the location (address) bar and press the "Enter" key, just like you type the url of a website to open a website.
If you see a warning then you can confirm that you want to access that page.

• http://kb.mozillazine.org/about%3Aconfig

mmm problem not solved.. another posible solution? Thanks anyway..

Try middle click instead of double click

I just fixed this on my Firefox 4. Try this:
1) Right-click on the tab-bar whitespace
2) Uncheck "Tabs on top"

Now, double-clicking on the whitespace acts as you would normally expect. For me, this opens a new tab.

this functionality just got removed in the newest FF update? middle click works but the wheel on my mouse sucks what was wrong with leaving double click active too?

BRING BACK THE DOUBLE CLICK TO OPEN TAB

Double-click still works for opening a new Tab in Firefox 7.0.1, you have something wrong with your Firefox installation.

Try the Firefox SafeMode to see how it works there.
A troubleshooting mode, which disables most Add-ons.
(If you're not using it, switch to the Default Theme.)

• You can open the Firefox 4/5/6/7 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.
  

Don't select anything right now, just use "Continue in SafeMode."

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