I'm somewhat surprised that Groups.io members who use Thunderbird,Bingo.
I almost never use the footer links in either individual or digest messages. Specifically because I inline reply with quote, as herein, and the footer links don't give me a copy of the original message to work with. So in individual messages I use Thunderbird's Reply button. In the rare case of replying to a Digest I probably would go on site.
I'm more and more persuaded that this is a Thunderbird bug and not aYou're probably right there.
Thunderbird does not encode the links in the mailto: Subject line atI don't think that's the problem. That is, I think Thunderbird is just showing you human readable form when you hover.
My experience is weirdly different though.
For me, if I'm reading the message in the preview pane (as I usually do) in either individual or digest, clicking on either the Reply to Group link or Reply to Sender, link gets me an open compose window, with the From, To, and Subject correctly filled in.
But, it also gives me a new tab on the main Thunderbird window, and that tab sits there blank with the tab label saying "Loading...".
Weirdly if I open the message into its own window I get inconsistent results, sometimes correct, sometimes absent To and Subject. So maybe the user's experience of this bug depends on some setting or view layout in Thunderbird.