Log Message:
Bump DIST_SUBDIR to v3: Use checksums of manually (firefox) downloaded
distfile. This is the same as before my previous commit, but into a versioned
subdirectory.

Automatic fetch from sourceforge will probably fail (I get three different
checksums for different tries), but I've also uploaded this to ftp.NetBSD.org,
so the failover fetches will work, eventually.

(Keep MASTER_SITES for updates.)