firefox-esr-dbgsym for 60.7.0esr-1~deb9u1 unavailable for stretch

Dear Maintainers,

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)


Best wishes,