I'm not sure if this is an xdg-utils or chromium bug.

After installing chromium, it automatically became the XDG "default-web-browser", used by
xdg-open, which in turn is invoked by many programs to display their HTML help/manual pages.

This shouldn't happen; only the user should be allowed to set the default browser.
I had to restore my preferences running "xdg-settings set default-web-browser firefox-esr.desktop"

Fortunately /etc/alternatives was not affected by this problem: chromium was added to the available
alternatives for x-www-browser etc, but the default user preference was not changed.

xdg-utils on Debian really ought to default to x-www-browser.

This would be better than an arbitrary default (which I cannot
change). Or sensible-browser (provided by sensible-utils) when
available; it has the advantage to dynamically be configurable
by the end user (who can use 2 different browsers in 2 different
contexts at the same time).