What is Shared Menus Framework?

Hi All.. ( I am still in search of the most active place online where 
people build and debug firefox on mac osx)

Like I posted here: http://forums.mozillazine.org/viewtopic.php?t=99955

I wanted to try to build firefox for mac from source. According to the 
directions stated here: http://www.mozilla.org/build/mac.html
I need to download a SharedMenusCocoa.framework from a third party 
(looks comercial) site.

I am just curious as to:
what is this framework,
why is it needed..
is it supplied by Apple? (doesnt look like it)
and why is it needed only at build time?

Thanx
Spundun