A lot of people seem to find the default Opera theme ugly. This was true for Opera 9.x but the latest skin on Opera 10.x is pretty sleek. So if you are a Firefox user and wish to skin your browser to give it the looks of Opera, you have two choices.
Fopera is lightweight and fully functional skin for Firefox 3.5 and recent, although it is still in progress. It features the same tab position as in Opera (tabs just below menu bar and above address bar), and most of the same images. Small icons is not yet supported but if people enjoy this theme they will be added along with other compatibility features, the theme creator writes.
Another theme is Operetta Advance for Firefox 3.6 and recent.
Both looks pretty much the same but I’m leaning towards Fopera. I suggest you to check out both.