We incorrectly stated in the Thunderbird Merges Completed post that Thunderbird 6.0 Beta (6.0b1) was available. Currently there are no Beta builds (with the exception of Tinderbox Builds) for Thunderbird 6.0. Those who had been on the Beta channel with Thunderbird 5.0 were updated to the release version of Thunderbird 5.0. However, I am uncertain if and when the Beta builds for Thunderbird 6.0 are released if an update to Thunderbird 6.0 Beta will be presented. Should have more information in the next few days.