Firefox 3.6.4 is a security and stability update to earlier versions of Firefox 3.6.x. The actual public release of Firefox 3.6.4 is targeted for May 13 but the developers have decided to provide an update to Firefox users who have been running beta versions and developer builds of Firefox 3.6.4. This update is already provided via the build in update checker of the web browser and will update Firefox to Firefox 3.6.4 Beta 3.
Users who are running a beta or developer version of Firefox 3.6.4 can go to Help > Check for Updates to display the information about the update with options to download and install it right away.

Firefox 3.6.x users who want to update to Firefox 3.6.4 right away because of the security and stability improvements need to install the beta version first and then perform the update check.
The beta version is available on the Firefox beta page.
The full list of bugs that are fixed can be viewed at Bugzilla. The bugs included two rated as blocker, 19 as critical and eight major ones. The total amount of bugs fixed in Firefox 3.6.4 is 189 across all supported operating systems.
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The 2 blocker bugs are not related to Firefox per se, but to Firefox’s building process (building of binaries for different OSes and locales). In other words: There was nothing really wrong in Firefox; there was something really wrong with the Firefox building process at Mozilla.
There is no need to install beta. Just go to Mozilla ftp site and download latest version.