OWASP Browser Security Project
The current version of Firefox can be downloaded here
Below is a list of some of the security features within Firefox and links to additional information:
Content Security Policy (CSP)
Strict Transport Security (STS)
Security Bug Bounty Program
Report a Firefox Security Bug
Please file a bug describing the security bug; be sure to check the box near the bottom of the entry form that marks this bug report as confidential. We encourage you to attach a "proof of concept" testcase or link to the bug report that demonstrates the vulnerability. While not required, such a testcase will help us judge submissions more quickly and accurately.
Notify the Mozilla Security Group by email and include the number of the bug you filed and a brief summary. If you cannot file a bug include the full details in the email and attach any proof of concept testcases or links. Mozilla Foundation staff and the Mozilla Security Group will consider your submission for the Security Bug Bounty and will contact you.