From what I've gathered on firmware release notes and from my own knowledge, they all use the exact same firmware because they all use the same Puma5 chipset. There is a new mode in the firmware called "World-Wide Mode" for the European models which essentially allows them to be used in North America as well I assume.
Unless otherwise noted, all firmware Fixes and New Features affecting the complete SB612x and SB614x family will be identified using the generic SURFboard Cable Modem (SBCM) naming convention. Any product specific changes will be noted with the specific product name, i.e. SB6120J or SB6120E.
The SB612xE supports Motorola's new "World-Wide" mode with dual certificate capability (BPI+), allowing the modem to register on a EuroDOCSIS or DOCSIS CMTS. In addition, the SB612xE with "World-Wide" mode enabled will scan for EuroDOCSIS-256QAM, EuroDOCSIS-64QAM, DOCSIS-256QAM, and DOCSIS-64QAM signals.
With the introduction of the SB_KOMODO-1.0.6.0-SCM01 firmware release for the SB612x and SB614x product lines, a firmware naming convention change has taken place. With this release and all future releases, the SB612x and SB614x products will share the same image file designated with the SB_KOMODO prefix.