[X4U] mbox problems

[nk]

I'm trying to migrate to Mozilla Thunderbird from Gyaz Mail. Gyaz has an 
Export >Unix mbox… function which I used on each of my mail boxes, and 
placed those resulting files where Thunderbird needs to see them.

Thunderbird shows the names of those mail exports in its Folders column, 
but won't show the mail they contain. If I click on one of them, I get a 
wristwatch, but nothing ever happens.

Any thoughts on what I did wrong here?

thank you!

nk