The hardware is based on their 6502-based board, with a change of CPU and logic chips. 

I already have a better 6809 board and peripherals, so I'm not that keen on getting the board for renovation. The firmware might be interesting, so I checked J.G. Harston's extensive website. It wasn't there. So I bought just the ROM data.

The EuroCUBE-65 manual documents the 16 different link-selectable memory maps. Two of them are for 6809 Flex systems. The ROM is enabled is the top 8K of memory space, but disabled in places to allow space for RAM and I/O.

E000-E7FF = RAM, 2K
E800-FFFF = ROM, 6K less 256 bytes
EE00-EE7F = I/O page, 256 bytes

I could not find an online 6809 disassembler, but did find many downloadable ones. I chose one that was Flex OS aware, as this is a Flex OS firmware ROM. 

The VIA, UART and RTC locations are taken from the manual, but the FDC locations are guess. The 2793 chip has four contiguous locations.

I can't spare the many hours it would take to do it completely but it is good enough to see where the RAM, ROM, and I/O devices go. I'll leave further work to keen Flex enthusiasts.

Most importantly, the firmware has not been lost forever.