I suspect that Benjamin has xul-ext-gnome-keyring installed, with which
Thunderbird 60 broke compatibility; see:
"xul-ext-gnome-keyring doesn't work with firefox >=57"
<https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=881506>
The assertion there, "In the meantime this still works for firefox-esr" from
December 2017,
is of course no longer true that 60 has become esr; and while there's not yet
any mention
of *Thunderbird* in that bug, it does appear to be the same sort of problem due
to
Thunderbird 60 having made similar motions to phase out the XUL extension
mechanisms--
which I guess is already known, seeing the notes in the Debian changelog like
this:
* [4a993c5] adding additional packages to Breaks with thunderbird
The packages calendar-exchange-provider and enigmail
xul-ext-sogo-connector aren't compatible to the webextension interface
and we need to add a versioned Breaks.
I had the same problems as Benjamin described with a GMail account,
but *also* had a similar "Thunderbird just never completes login" problem
with an Dpvecot IMAP server that I administer (also on Debian).
After removing the xul-ext-gnome-keyring package, Thunderbird 60 works with
both accounts :)
I guess the thunderbird package should add xul-ext-gnome-keyring to its
"Breaks" list?
