Before LOS (the Molecular's first operating system) customers were provided with a set of utilities to allow them to develop their own applications. These would be core images of low level functions such as disk and terminal handlers, interrupt handlers and simple arithmatic routines.
The Firmware typically occupied pages 1 to 4 in core. (Much of those original routines made there way into LOS)
Today we would expect Firmware to be stored as read-only - whether as protected memory or even ROM chips - and therefore uncorruptible. The Molecular 18 firmware was just as vunerable to be clobbered by a poor programmer's choice as any application.