I use Ubuntu 12.04 with various Firefox versions available. I am not certain I understand exactly what you are asking for, maybe an annotated screenshot would help

• How do I create a screenshot of my problem?

Also note that the current release of Firefox is 16, and so you should try to avoid using Firefox 13 as it is now old, unsupported, and not secure.

If you have used Firefox in Windows 7 or, if one has used Google Chrome Web browser, you shall know, what exactly I mean by having 'tabs-in-titlebar' , which incidently is not available in linux platform either by default or via an addon. Thank you.

I have opened three browser varieties including Google and Firefox both by default are showing browser tabs above the navigation bar, a third is showing browser tabs under the navigation bar.

I attach a screenshot, I am still unsure exactly what it is you want when you say tabs-in-titlebar.

On Linux the Firefox button is on the Tab bar and you usually have the title bar visible via the OS setup.
It is not possible to have the tab bar in the title bar on Linux or place other items on the title bar.
Only Firefox for Windows has that feature and only if the Firefox window is maximized you can have the Firefox button and the tab bar in the title bar.
On Windows the title bar is in the Firefox DOM (#titlebar), so it is possible to place other items on the title bar.

You can use this pretty add-on: Hide Caption Titlebar Plus.