You have to be creating a message in HTML for Insert to be possible. Are you creating a Plain Text message? If you hold the shift key when clicking the Write button is Insert there?

Airmail said

You have to be creating a message in HTML for Insert to be possible. Are you creating a Plain Text message? If you hold the shift key when clicking the Write button is Insert there?

Thanks, I didn't know about that. I found the problem. In Account Settings/Composition & Addressing, "Compose messages in HTML format" wasn't checked. Not sure how that happened. Keeping it checked was my default.

I have the same problem. An inserted image is present in the body of my letter, when I write it, but the image is absend as in sent letter (Sent Items folder), so as in received letter (I try this, sending letters between my two e-mail boxes). The letter is composed in html, In the previous Thunderbird version there wasn't such a problem.

Hi bvn123, This thread has been marked as the solved by the person who asked the original question (GaryK). Your issue may have similar symptoms, but it is likely a different cause/solution. You should use https://support.mozilla.org/en-US/questions/new where volunteers can get more details about your setup.

Because this thread is solved and more people might start using it for their own questions, I'm going to lock it.