Just update the line in FIRFirestoreSettings.mm file (I have not updated any pods)
From
ABSL_CONST_INIT extern "C" const int64_t kFIRFirestoreCacheSizeUnlimited =
Settings::CacheSizeUnlimited;
To
extern "C" const int64_t kFIRFirestoreCacheSizeUnlimited =
Settings::CacheSizeUnlimited;