I wasted an hour or so trying to import email files from a deleted IMAP email account. The import export tool wants to see a file with the .mbox extension somewhere in my local profile directory. However there is no .mbox file in my entire account profile! There are plenty of .msf files however and text files with no extension of the same name. It took quite a bit of searching for me to finally guess that those no-extension text files were indeed the .mbox files. I had to rename them to add the .mbox extension. Only then would the import and export add on tool actually succeed in importing those files into a local folder. Whatever has changed in the file layouts for thunderbird, is not expressed within the known tools that import and export these files. This qualifies as a bug, but I'm not sure for whom. Probably the import export tool? At the very least it needs to allow for the fact that the .mbox extension may not be present.