Button sizes
  16x16 (small-images toolbar button)
  24x24 (large-images toolbar button)
  32x32 (extension manager icon)
Colors
  Normal
  Hover: darker (?) color to match winstripe (16x16 and 24x24 only)
  Disabled (not necessary for Thumbs, because the button is never disabled)
  All 16x16 icons (etc.) should be in the same .png so I can use the -moz-image-region trick to prevent flicker on hover.