If you're attempting to upgrade to Firefox 4 on Mac and encounter this beautythen you'll need to perform the following terminal commands as an easy workaround.
cd /Library/LaunchDaemons/ sudo launchctl stop com.cisco.anyconnect.vpnagentd sudo launchctl unload com.cisco.anyconnect.vpnagentd.plist killall vpnagentd
Now attempt to install FireFox 4 again before issuing the following to reconfigure VPNAgent:
launchctl load com.cisco.anyconnect.vpnagentd.plist
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.