Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
Internationalization is the process of designing and developing a software product to function in multiple locales. This process involves identifying the locales that must be supported, designing features which support those locales, and writing the code needed.
It seems that nsMacCharset.cpp makes sense for the Classic Mac OS but is obsolete on Mac OS X, since Mac OS X doesn't have a platform encoding that'd vary by OS locale.
nsMacCharset.cpp should probably become similar to nsBeOSCharset.cpp