Thanks! Got passed that error by downloading the missing package for ubuntu.
It looks like now my gecko build is failing:
/home/ywei16/B2G_latest/B2G/prebuilt/linux-x86/toolchain/i686-android-linux-4.4.3/bin/../lib/gcc/i686-android-linux/4.4.3/../../../../i686-android-linux/bin/ld: cannot find -lrt
collect2: ld returned 1 exit status
make[8]: *** [libnspr4.so] Error 1
make[7]: *** [export] Error 2
make[6]: *** [export] Error 2
make[5]: *** [export] Error 2
make[4]: *** [tier_nspr] Error 2
make[3]: *** [default] Error 2
make[2]: *** [realbuild] Error 2
make[1]: *** [build] Error 2
make: *** [out/target/product/generic_x86/obj/DATA/gecko_intermediates/gecko] Error 2
I'm not sure what's going on, just two days ago the emulator built fine on this same machine, now everything is breaking... Thanks for your help John.