I've exported users emails using Google Vault. They've provided mbox files. I've gone through the steps of installing Thunderbird and the import extport tool add on.

When I import mbox files into Thunderbird, an empty folder is created under Local Folders and I can't see any emails. I've tested the mbox files are not corrupt by opening in a basic MBox Viewer, but we want to use Thunderbird so we have the ability to search these emails.