I recently rescued several years worth of emails from an old hard drive (2004, start of my company, through to 2015 when I started using Gmail for business) that's in a Mozilla Thunderbird profile, containing several mailboxes each with several folders. We're talking 11 Gb of raw data here, including attachments. Since moving to Gmail I haven't had to go back to that archive, but I'd like to have the option.

A quick look shows that it's a mix of SQLite and plain text so I should be able to figure this out, but I was wondering if somebody else had done this already.