Earlier, when tabbed browsing happened, all the browsers, which in those days was Opera and Firefox, did NOT have the tabs right till the top. The title bar would still show which in turn would give you a gap above the tabs to move the whole browser window.
So, when you're browsing full screen and you use your mouse to drag the whole window you'll click on the top bar and move it away (which in turn would restore the window to the non-maximized size).
Then Google Chrome appeared and started using the title bar to put tabs into. I think this also had to do with Windows 7 allowing this. (I don't think this was possible in Windows XP).
Other browsers, including Opera followed. However, Opera kept usability in mind and put a 1px gap between the top edge and from where the tabs begin. So, even in full screen mode you can control the whole window using the mouse.
How did Chrome solve this problem? Their tabs were tapered, so that you'd get a gap in between to tabs to control the window. In the case of Firefox, the tabs had more rounded corners to achieve the same thing. However, Opera tabs had always been the squarest, so they decided to leave the 1px gap.
In any other browser, which allows tabs in the title bar, when in full screen mode, try to control the window by clicking the top of any tab and you'll see that it cannot work. You'll only end up holding the tab and the browser window will think you're trying to detach it.
Hope this helps.