...But the thing that makes me really not trust the browser is that it doesn't matter how secure the original code is if the typical usage pattern of the browser requires users to perform insecure actions.
·Installing Firefox requires downloading an unsigned binary from a random web server
·Installing unsigned extensions is the default action in the Extensions dialog
·There is no way to check the signature on downloaded program files
·There is no obvious way to turn off plug-ins once they are installed
·There is an easy way to bypass the "This might be a virus" dialog
Read: How can I trust Firefox?