Some people such as myself have an old 1024x768px monitor
Yeah, some people. Other people who buy a modern monitor have few other options that getting 16:9 or 16:10 displays. With one of these, you're more than happy about Tree Style Tabs. In fact, I am even worried no such extension will be available for Firefox 4 when tabs are move to the window bar.
or a netbook with a 1024x600px monitor, and more and more web sites are designed to run maximized across the entire width of such a monitor.
Yes, for the small display of my netbook i do not use Tree Style Tabs, but here i have seldom more than four tabs open at the same time.