Opera is mainly credited with tabbed interface, but tabs appeared before this scandinavian browser added them in version 4 in 1999. Tabs were a logical extension of MDI interface (MDI stands for multiple documents in a single program window), as they provide quick navigation between documents you're working on. Just like Windows taskbar enables you to switch between programs.
and even before Windows 95, there was a browser InternetWorks, that dealed with multiple opened web pages in the same way.

Why they didn't patent it? Because concept of tabs is even older than that, used in various programs that can deal with multiple documents in single program window. In fact, Adobe DID patent some aspects of tabbed interface and sued Macromedia (they succesfully sued back), but those patents are seen as trivial and non-enforcable, as a lot prior art exists. Why weren't tabs patented by original inventor? Well, there was a time, when such trivial, albeit useful parts of user interface were not deemed worth of patenting it, unless you were Apple (they even patented Trash can:))