Did you use Thunderbird's native import feature or the add-on ImportExportTools? The native feature needs much improvement! If you have exported messages from a Apple Mail folder in thee mbox format you can manually import the mbox file via drag & drop into Local Folders in the Thunderbird profile folder. See also https://support.mozilla.org/en-US/questions/1483219

Drag& drop does not work for me - the folder just floats back to the Finder window. I installed the ImportExportTools extension. To be honest, I am confused about what is what - the native Import vs the I-E-T-Extension. I think I have tried both with always the same result - I can see the imported folder structure (1 folder per month ...) in the Local Folders section, but no messages. Then I created a mbox zip file - 1.94 GB (just under the limit) and the same thing happened ... Then I used the I-E-T-Extension Folder Context menu to import "All EML Messages from a Directory + Subdirectories" - this replicates the Apple Mail subfolder structure in the .../V10 folder - not very useful, but at least the messages are there, "All EML Messages from a Directory" or "Individual Messages" just report 0 files imported instantly. Repeating the same process on 2 additional years' worth of messages lead to the same result that I described in the original question - importing happens (Finder reports 2 lots of 10,000+ items being added to the Profiles folderstructure, but the disk space used only creeps up minimally) - the folder structure in the mbox files gets imported but the messages don't. So, I have now imported 3 years worth of Apple Mail .mbox export folders into the Local Folders of Thunderbird. 2 of these years' messages I can see and on the other 2 ( = the same year repeated twice into a different local folder) there are just the Apple Mail folders visible, but no messages.  ???? I am thoroughly confused !!! Eventually the process hangs with 174 of 12246 EML files imported, none nof them visible. Is there a way that I can make all messages in a folder tree "Unread", which would make them easier to spot in the sidebar?