I had the same problem with Ubuntu 14.4 on an AMD64 box;
Not sure how I ended up in that state, but it was obvious that, inside of firefox, the pipelight/silverlight plugin tried to load two dynamic libraries, and could not find them.
Both were attempted to be loaded as 32bit variant (more specificaly from a directory explicitly designated for 32bit libs.)
The related library files for 64bit architecture where present
p11-kit: couldn't load module: /usr/lib/i386-linux-gnu/pkcs11/p11-kit-trust.so: /usr/lib/i386-linux-gnu/pkcs11/p11-kit-trust.so: cannot open shared object file: No such file or directory
p11-kit: couldn't load module: /usr/lib/i386-linux-gnu/pkcs11/gnome-keyring-pkcs11.so: /usr/lib/i386-linux-gnu/pkcs11/gnome-keyring-pkcs11.so: cannot open shared object file: No such file or directory
I read about some hackish approaches like ubuntuforums about "p11-kit error Workaround WINE in AMD64 Ubuntu 12.04, and tried to install packages with similar names to get the libs.
The tricky part was that the libs are not in the packages where you'd expect them from the packages name, but slightly similary named packages.
So, I found out that these two packages
need to be installed to fix the two "couldn't load module" cases.
Note that this did not resolve a third error message line followed direcly to the the lines discussed above:
err:secur32:schan_AcquireClientCredentials Could not find matching protocol