Thanks and good point about the structured data.

I ask about mbox (one file system file per Thunderbird folder - e.g., one file named Inbox containing all its messages) or maildir (one folder per TB folder, containing one file per message) because it affects search using outside tools that don't understand that folder structure.

I'm wondering how efficient they are: When you search, does grep return an Inbox mbox file at a certain line number, or a maildir file?

It seems to be mbox, one file per Thunderbird folder.

Thunderbird itself seems to build some kind of an index next to mbox files. But finding the relevant email in TB's files makes it much easier to locate and open in TB itself (if it's needed). But I'd heavily prefer it to not be this way.