eww-browse-with-external-browser unexpectedly starts up librewolf instead of firefox

Hello all,

a recent update of Debian Linux (the only action I've done recently that
could have changed anything, I believe) has led to
eww-browse-with-external-browser, bound to & by default in eww, to start
up librewolf as the graphical browser instead of firefox.  Putting aside
any arguments about which browser might be better, I want firefox (at
least for the moment).

I cannot find any mention of librewolf in my Emacs configuration.  eww
invokes browse-url-secondary-browser-function which, in my
configuration, is set to browse-url-firefox.  However, invoking
browse-url-firefox actually starts librewolf!

Looking at the code for browse-url-firefox, it appears that the process
command that is invoked is "firefox URL".  If I type this directly in a
shell window (e.g. eshell) or a separate X terminal, I get firefox as
expected.  Yet M-x browse-url-firefox RET <some url> RET starts up
librewolf.  I am *so* confused!

Any suggestions of where to look for identifying this change in
behaviour?  I want browse-url-firefox to use firefox!  The hint is in
the name. ;-)

Thank you,
eric

PS - this is not a Debian issue, as far as I can tell.

-- 
Eric S Fraga via gnus (Emacs 31.0.50 2026-04-30) on Debian 13.5