Well considering that file->close only closes the current tab, I think it
might be a good idea to make this consistent with window border close. The
issues here are similar to the quit issue.

debate:
1. Close button close closes all tabs: Convenient when you have many tabs
open and just want to close the browser window, but not at all forgiving to
the user. Exposes the MDI nature of the app. Used as quit by most users so
its more of an application action than document action.

2. Close button closes only current tab: Slightly redundant with existing
tab close. Inconvenient to a power user because it requires them to close
each and every tab instead of groups of tabs. Xan has some anectdotal
evidence that suggests that this might be better for newer users. Is more
forgiving to user mistakes. Closing more than one tab simply requires the
user to hammer the window close button a few times (not that hard really).
Consistent with file->close.

The best argument imo is that making it close only the current tab is much
more forgiving to users who have multiple tabs already open and want to
close just one but out of muscle memory developed using most non mdi apps
tend towards the window border close button as oppose to the tab close
button and lose all their open documents.

Another option is to provide a menu entry in the tabs menu that closes the
window. Considering that the tabs menu will most likely only be used by
more skillful users, this might be a good comprimise. 

I'm comfortable with defering decision on this till post 1.0 if we want to
avoid flame wars :)