The release notes for Thunderbird 52.0 say that there has been a change to how photos are embedded in HTML message composition. That they will now use data: rather than cid: URIs for the image file. https://www.mozilla.org/en-US/thunderbird/52.0/releasenotes/ I'm running 52.5.0 yet my outbound messages are still using cid: URIs. Did the change get rolled back, or am I not understanding something about what that CHANGED note is saying? I didn't see anything in bugzilla, searching for "image" after April this year. I'm curious because I'm testing the response of a message email list archiver to the various ways of referencing images (http, cid, and now data) to make sure it handles each appropriately. I'd like to be able to choose which type to use in each outbound image.