The latest version of Firefox (Firefox 29.0) has brought round corners for tabs.
It can get pretty annoying if you can’t get used with the new user interface.
There is an addon that reverts the look and UI. You can download it from here: https://addons.mozilla.org/en-US/firefox/addon/classicthemerestorer/.
Install the addon, restart Firefox and you should be able to see the old UI with square corners and also with the orange Firefox main button placed on the left top corner.
The browser will look pretty much as before the new 29.0 update.