Got the following error when starting Firefox 3.0 on Ubuntu 9.04. - A complete removal and re-installation of Firefox did not fix the problem for me.
Could not initialize the application's security component. The most likely cause is problems with files in your application's profile directory. Please check that this directory has no read/write restrictions and your hard disk is not full or close to full. It is recommended that you exit the application and fix the problem. If you continue to use this session, you might see incorrect application behavior when accessing security features
The successful fix was removal of the hidden directory $HOME/.mozilla, as root user.
I suspect that the problem may have been caused by the file: nppdf.so, because this was the only file that required root access to remove. This file seems to have been installed by Acrobat Reader 9, which I downloaded and installed manually with gdebi (was not one of the regular Ubuntu apps in the Synaptic Package Manager). - Acrobat Reader still opens PDF files, but not inside the browser. It would be nice if someone could confirm if Acrobat was causing the problem.