Continuing with our effort to eradicate vulnerable versions of the Java plugin, we are now blocking it for all versions of Mac OS X. We had previously blocked it for old Mac OS X versions because they would not be updated anyway, and we were holding off on updating modern versions because of a bug in the Add-ons Manager that is now fixed in Firefox 12.
All users of Mac OS X 10.6 and above should have already been prompted to install this update. In order to avoid the block, please update immediately using Software Update. If for some reason you need to continue using an old version of the plugin, you can re-enable it in the Add-ons Manager.