The idea that a close button on every tab negatively encroaches on space for displaying the document title is really the result of another oversight in the way Firefox implements tabs. Firefox has no real mechanism for dealing with high volumes of tabs. On both the Mac and Windows, Firefox keeps reducing the width of every tab for each one you add until there’s just enough space to display the document’s proxy icon (favicon). Then it stops reducing the size of the tabs but keeps adding them outside the right-hand side of the window, where you can’t even see them.They run right off the window!

Again I’m going to have to invoke Safari’s tab implementation. Safari also reduces the size of each visible tab as more are added, but there’s a logical limit. Safari sets a minimum width for this reduction (about 80 pixels) at which point it stops shrinking the tabs and adds a disclosure arrow with a list of titles for those documents that can’t be displayed in the tab bar. This ensures that every visible tab will have enough space to display a close button and a useful portion of the document title. For a 1024×768 display this means you have about 12 to 13 tabs visible before they start getting stacked in the menu list.

Ironically, displaying page titles is probably of far lesser importance in Firefox since it also displays the site favicon on every tab (which, also pretty ironically, takes away space from the document title).

I still hold that the single tab close button metaphor seriously messes with the document interface model users would typically see in either Windows or the Mac OS.