Considering there is zero source history included here, neither for firefox, nor the amigaos changes, how does git help with the documentation for the code that has already been done ?
I assume the original code where they started from is available and then the final released code. You can import both in git with the second over the first and you then can easily see the OS4 changes. For a programmer worth it's name and familiar with the Amiga(OS) API that should already tell a lot even without documentation. You should even be able to do this in subversion but git will allow you to split up the big patch in smaller patches much easier. The latter does need quite deep knowledge of git features as history rewriting etc. though.