To diagnose problems with Thunderbird, try one of the following:

• Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.
• Restart the operating system in safe mode with Networking. This loads only the very basics needed to start your computer while enabling an Internet connection. Click on your operating system for instructions on how to start in safe mode: Windows 10, Windows 8, Windows 7, Windows Vista, Windows XP, OSX

If safe mode for the operating system fixes the issue, there's other software in your computer that's causing problems. Possibilities include but not limited to: AV scanning, virus/malware, background downloads such as program updates.

Hi Matt, tried both. I started Thunderbird with the addons disabled, there was no change. Started win 10 in safe mode and that did not change it. What happens is that when I have an html email I received such as the one below and I click on Forward, the forwarding window opens and the email looks like the second image below. It never did that before. Thanks. Joe

What anti virus are you using?

I can see what is happening, but I am at something of a loss to work out why. as reply/forward creates a temp file I can only assume something is messing things up at that point. Normally O/S safe mode disables anti virus, but that is not necessarily so with defender.

Hi Matt, I have AVG Internet Security and Malwarebytes Anti-Malware Home (Premium) installed an running all the time. Joe