I get tons of these types of errors every time I run gdb on my firefox these days: warning: Could not find object file "/Users/ehsanakhgari/moz/mozilla-central/obj-ff-dbg/security/build/tmpIifklV/derenc.o" - no debug information available for "derenc.c". Is this a regression from the recent NSS changes?

That comes from bug 648407. The only trick I can think of would be for expandlibs to check if the .o files it extracts from .a archives exist alongside.