I got in the habit of having multiple windows open when I used Netscape and was on dial-up.  Renderosity was so slow that you could grow old and die waiting for product pages/forum threads to open.  So I'd open 20 or so pages at a time, whether browsing the forum or the Marketplace.  By the time I was finished reading the first page, the next would be open. 

When I switched to IE, I found it really annoying that pages in new windows would "pop to the top" when they finished loading, interrupting my reading.  (Netscape didn't used to do that.)  Tabs fix that problem. 

Even though I'm have broadband now, I still go through the forums and MP opening all the pages I'm interested in at once, then slowing reading through them.  IME, it still saves time.