The current HarfBuzz codebase is versioned 2.x.x and is stable
and under active maintenance. This is what is used in latest
versions of Firefox, GNOME, ChromeOS, Chrome, LibreOffice,
XeTeX, Android, and KDE, among other places.
Prior to 2012, the original HarfBuzz codebase (which, these
days, is referred to as harfbuzz-old) was
derived from code in FreeType, Pango, and
Qt.
It is not actively developed or
maintained, and is extremely buggy. All users of harfbuzz-old
are encouraged to switch over to the new HarfBuzz as soon as possible.
To make this distinction clearer in discussions, the current
HarfBuzz codebase is sometimes referred to as
harfbuzz-ng.
For reference purposes, the harfbuzz-old source tree is archived
here. There
are no release tarballs of harfbuzz-old whatsoever.