One of the reasons. I think tabs were actually a bigger deal back then. Although Opera had them first, Firefox was the one that put them front and center. Back then, Opera was still championing a gesture-based interface, and Firefox felt a lot more solid and polished.
Opera had little miniature windows inside the main window. A small difference, perhaps, but tabs are simpler to manipulate at the cost of, perhaps, being less flexible (you can't resize tabs and move them around like you can windows).
Actually this is incorrect -- the windows you're referring to were part of the earlier MDI version of Opera. In 2000 Opera 4 came out with tabs very similar to what FireFox would have in its first release four years later (and its predecessor Phoenix had in 2002).
