Tabs = Windows

Have you ever wanted to look at two tabs in your browser at the same time? As far as I know, FireFox doesn't have any function to "detach" tabs.

If you could detach, resize and reposition browser tabs, they would mimic the function of desktop windows. You could have a "virtual desktop" of sorts running in your browser.

Being able to use tabs as windows would further the dependence of the user experience on any specific OS. I wonder if this is why Microsoft was the last to enable tabbed browsing.