Mac Firefox 3.6 behaviour with local files

I would like Mac Firefox 3.6 to use the same setting for opening local 
files as it uses for opening new web pages.  In other words, it should 
open the local file in a new tab, not a new window if I have that 
setting enabled in Preferences > Tabs.  Why have it not open directly to 
the local file if it is not running already, instead of opening the 
window on the home page, and forcing me to double-click the local file 
again, only to have that open in a new window?
Why does this behaviour happen only in Mac Firefox and not in Windows 
Firefox?