With the release of Firefox version 4, tabs have been moved to top position similar to Google Chrome. But if you have been a Firefox user for long time, there might be some difficultly adjusting to the new layout of tabs which are now moved to top position above the navigational bar. But in case you want to disable the tabs on top feature and want it in the same position as previous versions of Firefox, then you can do it in simple step.
To disable the tabs from the top position, click on the Firefox button on the top left corner. From the menu, click Customize-> Tabs on top. Uncheck the box against “Tabs on top”.
Now you can see tabs moved to the bottom position.
If you want to restore the tabs to the top position, just check the option in the step mentioned above.