Tinderbox: s/Thunderbird/Thunderbird3.0/g

As you might know already, Thunderbird development since Beta 1 has switch over to following the mozilla-1.9.1 branch of mozilla-central. Now that we are getting ready to bring up new builders building against mozilla-central again, it was decided to split them up into 2 distinct Tinderbox trees.

Thunderbird3.0 will be tracking releases/mozilla-1.9.1 and Thunderbird will keep on tracking mozilla-central. Shortly, all existing builders will be moving to the Thunderbird3.0 Tinderbox tree, and as the new builders come online, they will start showing up in the Thunderbird tree.