With Firefox 29.00+ versions, the below solutions might not work. Firefox has also changed the theme. If you want to get the old theme back and also want Tabs to be below the Address-Bar then the solution is to use Classic Theme Restorer add-on.
Firefox has made lot of changes in the past few updates, one of which was moving Tabs on top. How to move them back below the address bar, like they used to be in the past?
Open Firefox: right click on blank space in front of the last Tab. Here, uncheck the following option:
Tabs On Top
That’s it: tabs will now show below the address bar.
Open Firefox and type the following [without quotes] in the address bar:
If a warning message shows up, click on:
“I'll be careful, I promise”
In top-left corner, a Search Bar will show up. Type the following in it;
An entry will show up. Change its value to False.