the Macintosh File System. The first file system used by the Macintosh when it was intoduced in 1984. Balky and slow, it was abandoned with the advent of the Mac Plus and the Profile hard drive. It was abandoned for several reasons, the most important being a) it did not have a multi-level directory capability (directories could only be one level deep), b) had 32 file-per-directory limit and c) could not address more than 5 megabyte volumes in any reliable way.
It was replaced by HFS which was replaced by HFS+. Support for reading MFS formatted volumes was finally dropped in MacOS 8.1, over 14 years after it was introduced.