To make htm attachments displayed inline, you have to change a preference. From the TB 68 release notes:

"Textual attachments (plain text, HTML, XML, etc.) are no longer displayed inline. Toggle preference mail.inline_attachments.text to [true to] display them inline again."

Tools/Options/Advanced/General/Config. editor, double-click a preference to toggle its value.

Thanks very much for the assistance. Seems like an important issue to fix by default. Your fix has seemed to work and fix the issue.

Unfortunately this fix has caused another issue with thunderbird. I applied the fix suggested by changing "mail.inline_attachments.text" in the config editor to true. This did solve the issue I was having with html emails not displaying properly. However after I changed that setting, now when I attempt to forward an html email, instead of forwarding the email content, Thunderbird forwards the body of the email as an attachment named "attached Message part". I frequently forward saved emails that are used often by customers. This is an issue that is happening only on some html email - some forward properly without the attachment - some foward with a blank body with the attachment.

So, is there a way to still benefit from the original fix with the config editor while still have my email forwarding work properly for all emails?

If you right-click the message, Forward As/Inline, does that change how the message is forwarded? The default is set in Tools/Options/Composition/General.

I checked Tools/Options/Composition/General and attachments are set to forward inline. No, when I right click on the message and choose Forward As/Inline, it does not change the problem. The resulting forwarded message shows up with the html body missing and an attachment named "attached Message part" is visible.

This should not be happening given the settings. It should be forwarding inline but it is not. Please advise on any other options to fix. Thanks, Ben.

I believe I may have solved this issue. About a month ago I changed a link in my signature which points to the url for the company logo. I simply updated the link from http: to https:. Ever since I did that the forwarding of any message that contains the https: url in the signature has caused me problems. I tested this by changing the signature url back to http: and then taking an identical email other than that url and completing some side by side A/B testing. The email with http in signature fowarded fine without the weird attachment. The same email with https signature caused the strange attachment "attached Message part".

Any idea why a image url for signature logo with https: would cause this issue within Thunderbird?

I don't know why the https would cause the issue, unless the server doesn't actually support https. It takes a fair number of steps to enable it, and you can test the results.