Probably the longest test run ever for a Firefox maintenance release, but then again, it's unusually interesting this time.

• Firefox 3.6.4 provides uninterrupted browsing for Windows and Linux users when there is a crash in the Adobe Flash, Apple Quicktime or Microsoft Silverlight plugins.
• If a plugin crashes or freezes, it will not affect the rest of Firefox. You will be able to reload the page to restart the plugin and try again.
• Fixed several security issues.
• Fixed several stability issues.

For a complete list of changes, see Bugzilla. You can download the binary and upgrade your existing installation, the auto-update system should be triggered automatically (or from the Help menu).

Update: Skipping 3.6.5 all together, Firefox 3.6.6 has been released, which increases the time-out counter to determine crashed plug-ins. Previously this was set too low, which was causing problems for older machines running Firefox.