I can't reproduce this. Is your system updated, or did you install firefox on an outdated system? You might want to use strace to find out why XPCOM can't initialize.

I'm not sure why or how but yesterday I managed to install a bunch of 32 bit libs on my 64 bit machine. (It killed everything inclusing bash.) I thought I reinstalled everything 64 bit. I wonder if this is related...

strace attached.

xulrunner is up to date. And I just checked libx11 (libx11-1.3.2-1).

strace shows a ton of open()'s failing on "/usr/lib/xulrunner-1.9.1/*" files with ENOENT (No such file or directory).

Why is my xulrunner missing all the .so files from that directory?

37 fails

[code]open("/etc/gre.conf", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/home/goodi/.gre.config", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/home/goodi/.gre.d", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/lib/libnspr4.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/lib/libplds4.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/lib/tls/libnspr4.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/lib/tls/x86_64/libnspr4.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/lib/x86_64/libnspr4.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/tls/libnspr4.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/tls/x86_64/libnspr4.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/x86_64/libnspr4.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libX11.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libXrender.so.1", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libcairo.so.2", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libdl.so.2", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libfontconfig.so.1", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libfreetype.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libgcc_s.so.1", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libhunspell-1.2.so.0", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libjpeg.so.7", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libm.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libnspr4.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libnss3.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libnssutil3.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libplc4.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libplds4.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libpng12.so.0", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libpthread.so.0", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libsmime3.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libsoftokn3.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libssl3.so", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libstdc++.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/libz.so.1", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/tls/libpthread.so.0", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/tls/x86_64/libpthread.so.0", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/xulrunner-1.9.1/x86_64/libpthread.so.0", O_RDONLY) = -1 ENOENT (No such file or directory)[/code]

The -ENOENTs are because the linker scans a set of paths to find its libraries. Firefox and xulrunner have been compiled with /usr/lib/xulrunner-1.9.1 compiled as library path, so every library is looked for at that place first.

As for your log, this one is suspicious:
open("/usr/lib/libplds4.so", O_RDONLY) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0000\n\0\0004\0\0\0"..., 832) = 832
close(3)

When a valid library is opened, it shows open, read, fstat, mmap, etc. When an invalid library is opened, it shows open, read, close, which is because the library does not suffice. It looks like your nspr package is still i686.

Ah. That did it. Thanks and my apologies for the false alarm. I got smart and fixes them all.

pacman -Qq | while read i ; do pacman -Qi "$i" | grep -q i686 && pacman -S "$i" ; done