Solution for Firefox 4 installation failure on Mac OS X due to 'libsmime3.dylib is in use' error

If you're attempting to upgrade to Firefox 4 on Mac and encounter this beauty

then you'll need to perform the following terminal commands as an easy workaround.

cd /Library/LaunchDaemons/    sudo launchctl stop    sudo launchctl unload    killall vpnagentd

Now attempt to install FireFox 4 again before issuing the following to reconfigure VPNAgent:

launchctl load

I've seen some other posts online suggesting that the killall command is sufficient, but that didn't work for me because vpnagentd was setup to automatically restart. The process I outlined here prevents that.