I'm in the process of trying out Thunderbird for my email. I've gone through the process of getting the mbox file from the mbox package, and copying it to Thunderbird's profile folder.
So far I've been able to ge the emails in the Inbox folder to appear. I have quite a few folders to add.
It looks like I can add subfolders to Inbox or to another location, then extract the mbox files from the subfolders (I hope!) to the appropriate location, then change the name to the folder name that I made. Given the number of folders, that seems like something that could be automated if I know what I was doing.