You can do that in vanilla Firefox 3.6 and I'm pretty sure it's worked since at least 2.x
The whole point of tab candy is to fix this deficiency.
You can't drag a tab into window without tabs (only 1 tab hence no tab area). You also cannot drag a window (only 1 tab) into another window.
FF does have some subset of Chrome's functionality, but you cannot use it like Tab Candy to easily organize sets of tabs. Which is the whole point of the post.