>> Sure, grouping them in one frame is nice, but not more.
No, it's more. At least to me. I don't mind if it isn't to you.
When I first started using Firefox, I also used IE6. There was a huge difference between the two, and 95% of that difference was the tab feature. Perhaps I browse differently from you. An example of its use is browsing forums. I middle-click on 3-5 threads, then read through each one in its own tab. Without tabs you either have to read one, then go back and click another, or open each in a separate window. Then when you alt-tab between your browser and the other two applications you're running, you have to do it 5-7 times to get back around. Neither solution was acceptable for me (when compared with the tab solution).