Right Click on the plus sign (which creates a new tab) and select “Tabs on Top”.
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I don’t know what happened after i installed an addon on my firefox 15 and after it restarted, firefox performed an automatic update to firefox 16. Then i realized the tabs came down from the title bar and looked somewhat like this. [see image below]
This was little bit annoying for me because i felt that a lot of space is being wasted on the title bar, and i was not quite sure if the tabs were previously on the title bar or not. I got more annoyed because i couldn’t find out what caused the change.
All of a sudden, i found that, there is an option called “Tabs on Top” which enables to have tabs on the title bar, like chrome. (I dont like chrome for some reasons!)
So, just right-clicked on the plus sign (which usually creates a new tab)
and clicked “Tabs on Top” and it was done!
[See image below]