I would not see it as a bug. I would however see it as worrying that you have two firewalls active. Avast has one and now you have installed another.

I'm sorry, but you are wrong. I am running one anti-virus and one firewall.

First, Avast Free is an AV that does not include their firewall, which is available only with a paid upgrade. Avast Free is an anti-virus only, and IT security experts recommended that you use a firewall and an AV.

Secondly, I have not "installed" a firewall. Tinywall is a simply a "front end" utility that makes it easy to configure and manage Windows Defender Firewall, which is an integral part of Windows 10 that can be enabled or disabled. WDF is not easy to configure and manage without a third-party front-end. Like Mozilla products, Tinywall is an excellent free product.

I am not accusing Mozilla of owning the bug, but rather suggesting that the bug involves the interaction of Avast Free AV, Windows Defender Firewall and Mozilla Thunderbird.

Does this not affect other networking programs? Browsers, dns, ntp and ftp clients? Or is Thunderbird your only non-default program?

I think that there is a lesson to be learned here - and this will be useful for other non-Microsoft applications. Windows Defender Firewall - when managed via a 3rd party front-end like TinyWall - may not list all applications/executables that are trying to access the internet and need to be white-listed. That would place the "bug" in the lap of Microsoft and/or Avast, but they probably don't care.

Remember that the solution - which I found on an obscure French website - involved someone figuring out that white-listing Thunderbird itself was not enough to allow the sending and receiving of email messages. They obviously dug deeper and found that an Avast executable was the problem. Probably the one that checks messages that are being sent and received for viruses and appends the little html blurb to the end of the text of outgoing messages. And as best I remember, Avastsvc.exe was not presented in the list of candidates for white-listing, and some smart person isolated the problem and presented the solution. I'm just passing it on.

I have had no other internet/personal network problems with other applications or functions since activating Windows Defender Firewall and managing it with TinyWall yesterday. I now consider Free Avast plus Windows Defender Firewall (managed with Tinywall) to be the best free Windows internet security combination.

--- Personal note ---

I am not a Microsoft fan, but in 1984, I chose to purchase an IBM PC for $5,000 and was audited by the IRS for deducting it as a business expense to help manage my aerospace sales territory. I viewed Apple products as "computer nerd toys" and expected the PC-DOS and MS-DOS machines to dominate the market - especially for businesses. And that is exactly what happened.

I retired from Windows, DOS and OS/2 desktop support and Novell Network administration in 2001, and now live as a retiree expat in Panama on a limited income. Although I use the Windows 10 OS that came with my laptop, I spend most of my computer time using FireFox, Thunderbird, OpenOffice and other freeware. I dual-boot LibreELEC and Linux Mint on my little Intel NUC mini-HTPC, and have an Android tablet and smart phone. I have no interest in bloated and expensive commercial apps

Thanks to everyone who helps people find solutions to problems on open forums like this one.