Hi Chip,
In Mozilla Thunderbird you can go into the address book and then the Tools -> Export menu to get a version to work with. Export your address book to a comma separated csv file. You can then use Microsoft Excel to work with the csv file and break it up into multiple csv files. If you do not have Microsoft Office you can download a free version called
Open Office that will have the same functionality. Make sure you keep the same formatting within the csv files. When you are done you can go back into your address book and use the Tools -> Import menu to pull the data back into Mozilla Thunderbird.
Welcome to Email Questions! -Raymond