... or the day I fixed my mail clients running on Debian/stretch.

First of all mutt failed to open html mails or the html multipart stuff in Firefox. I found some interesting hints in a recent thread on debian-user. So now my "~/.mailcap" looks like this:

text/html; /usr/bin/firefox --new-tab %s;
text/html; /usr/bin/elinks -force-html -dump %s; copiousoutput

and I added the proposed "~/.muttrc" addition verbatim:

bind  attach  <return>  view-mailcap
alternative_order text/plain text/html
unauto_view *
auto_view text/html

For work related mails, where the use of html crap mails is a sad reality I can not avoid, I stick to Icedove. But beside of the many crashes everyone encountered recently it also crashes when I try to reach "Preferences -> Advanced -> Config Editor". So no chance to adjust the handling of http/https links in the UI. Luckily that configuration is still text, well XML, in a file called mimeTypes.rdf in in the profile directory. So I manually replaced "/usr/bin/iceweasel" with "/usr/bin/firefox" and a restart later clicking on http and https links works again. Yay.