Opera wasn't the first browser with tabbed browsing by far. There were a lot of Windows browsers that embedded the Trident engine, and some of them used variants of MDI for browsing. A few of them used tabs, the first well-known one being NetCaptor. Interestingly enough, it put the tabs on the bottom, which makes a bit more sense from a Fitts' Law perspective than putting the tabs near the other controls. This was a few years before Opera switched to tabbed browsing.