XPCOMGlueLoad error for file /usr/lib/firefox/libxul.so:
/usr/lib/libnss3.so: version `NSS_3.79' not found (required by /usr/lib/firefox/libxul.so)
Couldn't load XPCOM.
Updated the next following packages:
pacman -S nss zlib sqlite
Then firefox works!
Taken from: https://unix.stackexchange.com/questions/158329/unable-to-launch-firefox-keeps-on-crashing