It is also possible to easily copy either the e-mail address itself, or the whole mailto link, that is given by the Reply to Sender link at the end of messages.

In Thunderbird, if I have focus on the Reply To Sender link, one of the options that comes up if I bring up the context menu is, Copy email address, which does exactly what you'd think it would do, and gets only the e-mail address out of the mailto: link.  You can then manually compose a new message and paste that email address into the To: field.

Also, oddly enough, if I attempt to directly activate the Reply to Sender link in Thunderbird, I get a new tab for composing that says, Loading, and never loads.  Yet, if I gain focus on the Reply to Sender link, bring up the context menu, and choose Compose message to, a new composition window, rather than tab, opens right up with the e-mail address of the sender pre-populated, but without the subject being pre-populated as it would were the Reply to Sender link working as designed.

I do not have any issue with the Reply to Sender link in Outlook 2016, eM Client, or Gmail webmail.  This is a Thunderbird bug.  But now that I've done some additional resource it definitely is easy to get around and you still use the Reply to Sender link, but don't activate it directly, but bring up the context menu and go from there.

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Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042  

If you think that you can think about a thing, inextricably attached to something else, without thinking of the thing it is attached to, then you have a legal mind.

        ~ Thomas Reed Powell