To obtain the newest update now:

Click the Help menu > Check for Updates.

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Fixed in Firefox 3.6.4
MFSA 2010-33 User tracking across sites using Math.random()
MFSA 2010-32 Content-Disposition: attachment ignored if Content-Type: multipart also present
MFSA 2010-31 focus() behavior can be used to inject or steal keystrokes
MFSA 2010-30 Integer Overflow in XSLT Node Sorting
MFSA 2010-29 Heap buffer overflow in nsGenericDOMDataNode::SetTextInternal
MFSA 2010-28 Freed object reuse across plugin instances
MFSA 2010-26 Crashes with evidence of memory corruption (rv:1.9.2.4/ 1.9.1.10)

Via: http://www.mozilla.org/security/known-vulnerabilities/firefox36.html

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The Mozilla Foundation has released Firefox 3.6.4 and Firefox 3.5.10 to address multiple vulnerabilities. These vulnerabilities may allow an attacker to execute arbitrary code, cause a denial-of-service condition, obtain sensitive information, or conduct cross-site scripting attacks. Some of these vulnerabilities also affect Thunderbird and SeaMonkey.

Source: http://www.us-cert.gov/current/index.html#mozilla_releases_firefox_3_62

Also note they added a new feature that sandboxes plug-ins that may cause issues. http://download.cnet.com/8301-2007_4-20008495-12.html