Amazingly, there is virtually NO info on the net about this. It is a very serious problem for photographers like me who like to send clients an e-mail with embedded jpeg images. It is also a big annoyance when sending embedded screen captures....
When you attach a jpeg to a Thunderbird e-mail, the file content and quality is preserved. However, when you embed the same image (even a screen capture via cut/paste) Thunderbird seems to be recompressing the image at a VERY low quality. In order that my embedded jpegs appear decent, I have to save them at a very high quality (level 10 in Photoshop). This is presumably because a level 10 jpeg can withstand further compression rather well.
Try this (as I believe this is related to the same issue): Take a snapshot of a small application window (ALT+PRINT SCRN). Paste that into a thunderbird e-mail. You should immediately notice terrible jpeg artificats due to heavy compression. Paste the same clipboard content into Photoshop and you won't see any loss in quality.
Is there some hidden setting where I can tell Thunderbird to either avoid recompressing the embedded image, or to do so at a much higher quality?