Sure, there might only be six people in the world using the Opera browser, however you do have to admit those six people have a pretty nice looking theme for that browser of choice. Would you like to give Firefox the look and feel of Opera 10? If so, read on.
Both the Operetta and Operetta Advance themes for Firefox will give you that Opera look and feel you have been looking for. The main difference between the two themes is that Operetta has the tab bar in the usual location (under the bookmarks) and Operetta Advance will move the tab bar to the top of the browser (on top of the menu). So you have two choices to work with, depending on where you want the tabs bar.
Here is what Operetta looks like in action.