firefox-esr-dbgsym for 60.7.0esr-1~deb9u1 unavailable for stretch
As far as I see in debian-debug, the debug symbols of firefox-esr for
60.7.0esr-1~deb9u1 is not available for stretch (I mean, in stretch-debug).
There is a package of debug symbols named after 60.7.0esr-1~deb9u1 in
the pool, but it might be not for stretch.
(Beware that it might be a bug in the system, not just for a package, so
it is reasonable to dig out the reason)