It seems, they are currently destroying Thunderbird and it might be not long in the future when it is no longer usable well. I’ve been looking for an easy to use and stable mail client where the developers not constantly are changeing stuff just for fun and to annoy end users.

It may be that Claws Mail is an appropriate ersatz. Or maybe Sylpheed.

Both probably don’t support HTML Emails. Claws has a plugin that makes viewing HTML emails possible. It uses the Dillo-Webbrowser. That one is very light and small and fast but doesn’t support Javascript and maybe not all of CSS. But do you need such stuff in an email client?

Here some more info about claws in german