Today Firefox started crashing whenever I'm trying to open the preferences editor, and also often during normal navigation. If I start Firefox from a terminal, when it crashes it returns the message: Attempting to load the system libmoon.
It seems you've run into an instance of this bug: https://bugs.launchpad.net/ubuntu/+source/moon/+bug/538796
The workaround for now seems to be removing the moonlight browser plugin:
apt-get purge moonlight-plugin-core moonlight-plugin-mozilla
This is a common issue caused by Moonlight plugin. Uninstall moonlight-plugin-mozilla to solve your problem.
sudo apt-get purge moonlight-plugin-mozilla
If you still need Moonlight, then you could try the 3.0 preview version from: