You can simply use the File ► Save as.. function in Thunderbird email client to save older MOZEML email messages to EML format. It seems that latest Thunderbird no longer mentions the MOZEML format, but should open it just fine.
Convert Mozilla Thunderbird mail message to Email message.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert mozeml to eml on Windows
An easy to use e-mail client from Mozzila
Convert from mozeml file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.