When using mutt with mailto: From and Fcc are holding wrong values

[Martin]

Hello everyone,
I'm using mutt on Debian with several accounts and Firefox as a
browser. When I click on a mailto: link it opens a new terminal with
mutt and from all I see it does pick up my muttrc correctly, but the
new email has "Fcc:" as ~/sent and "From" as myuser
<myuser at myhostname>.

Same happens when I start it like:
mutt mailto:foo at bar.net?subject=foo

Also tried to pass one of my accounts with the -f flag.
I would expect mutt to select the first or any of my accounts to use
for the Sent-folder and From-Address.

Can you give me advice how to use one of my regular accounts for
composing emails via mailto links?

Mutt 1.10.1 (2018-07-13)
System: Linux 4.19.0-5-amd64 (x86_64)
ncurses: ncurses 6.1.20181013 (compiled with 6.1)
libidn: 1.33 (compiled with 1.33)
hcache backend: tokyocabinet 1.4.48

Best regards,
Martin