While surfing around the web we found various different firmware versions (Click on the version numbers to download firmware):
Mitsumi CR-4809TE version 1.0E:
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Mitsumi CR-480ATE version 1.0E:
AOpen CRW2440 version 2.06:
Philips CDRW1610A/CDD4911 version P1.4:
Source: The firmware page