Mozilla last week released, not one, but two new versions of the Firefox browser. Version 3.6.7 was released on July 20, and was quickly followed by version 3.6.8 on July 23. The first release, 3.6.7, fixed a number of stability bugs, and 14 security vulnerabilities; unfortunately, as sometimes happens, these fixes introduced another bug, which caused crashes on some pages using plug-ins. This bug was fixed in the second release, 3.6.8, which is obviously the one you want, and the one you will get using the built-in update mechanism.
Release Notes, with more detailed information, are available for both versions, 3.6.7 and 3.6.8. Installation binaries for all platforms (Mac OS X, Windows, and Linux), in your choice of more than 70 (human) languages, can be downloaded here.