Each Mail file has a companion MSF file. The MSF files are index files which contain your header information.
The SBD folders are the subfolders of the main accounts. If these folders are empty, then it is safe to delete them.
In view of the fact that you are coming from Thunderbird to Netscape, Thunderbird creates an SBD folder for each Mail folder in an account. However, Netscape does not, so if you are now using Netscape, check each SBD folder to ensure that it contains no files, and if not, the folder can be deleted...
The files with no extensions, i.e., Inbox, Drafts, Templates, Sent, etc. contain the Mail.