Thunderbird is not designed to work with dropbox in That manner, nor is it supported like that. It works much better with a sneaker.net if you insist on taking your data with you. https://portableapps.com/apps/internet/thunderbird_portable

If you want Thunderbird to function as designed place you profile folder in it's entirety on a local drive, even those half baked synced drives like Google and Microsoft offer will ultimately fail and corrupt your data. There is plenty of evidence of that in this forum.

Is it this version? Quite possibly. No one tests Thunderbird on anything by a local device and in the past this has led to issues on network shares. You are going one step past a local network share. Or it could be your store file has just passed the threshold in size to not make it download to the local disk before Thunderbird gives up trying to access it.

Dear Matt:

Thank you for the answer. My local folder is about 45 Gb. Is it too large? If I reduce the inbox file size, will it help?

I have been using the local folder in dropbox for many years (and it still works on my other computers). If it is working on an earlier version of thunderbird, where can I find earlier version of thunderbird to download?

I am sorry, I should have titled the question as "local folders are empty"

I eventually solved this problem by installing the same version of thunderbird as my old computer and copied over the entire thunderbird directory from the old computer. This gives me the same profile as the old computer and now I can see the contents of my local folders.