Red: Stop! The tree is closed. Green: Go! The tree is open. When there's bustage, the yellow light will be lit as well (it always accompanies either the red or the green light).

If you contribute to Mozilla (or have a Tinderbox of your own), this Widget will help you watch the tree so you know when to check in and when there's trouble.

There are a couple of other features as well, like a pop-up list of the busted platforms; and now with version checking!. Auto text placement. Read the "Read Me!" and check the preferences.