KWin does (did?) this as well. I think the biggest problem is that if you open a new "window" it doesn't know that you want it to show as a new tab on the same

KWin does (did?) this as well. I think the biggest problem is that if you open a new "window" it doesn't know that you want it to show as a new tab on the same window. I think that this is largely because X doesn't actually know which window opened the new window. I used the KWin tabs a fair amount, but they didn't fill the same use case as browser tabs. It would be interesting to see this implemented, of course you would also need to seriously patch the browser.