I found that Firefox fails to build if you have an existing 
/usr/include/sys/statvfs.h leftover from an older install/snapshot.

I was going quite mad trying to figure out why it was failing. 
Eventually I found that ls -lR in /usr/include helped me find all the 
files that were left over and weren't upgraded from my distribution build.

I removed these two files (based on timestamps):
/usr/include/sys/statvfs.h
/usr/include/sys/fstypes.h

After that I did a clean build of firefox-gtk2 and it worked.

BTW I'm tracking the -rnetbsd-2-0 branch.

Regards,

George