Sorry if this has already been mentioned, but the position for the close box for each tab should appear in the menubar at the far right. This would fit in with the convention used by all Windows MDI apps and also the one I've seen on Linux (Opera).
[] title - Mozilla {Build ID: 2001102808} _- X
| File Edit View Go Bookmarks Tasks Help x
The X on the menubar would need to have a tooltip (close tab) to
indicate the purpose of this.
