Wow, what a relief.
Most of the time I see a logo redesign, it's a mess that makes the product more difficult to recognize, and makes things look more generic.
Not so this time! Immediately you feel that the Thunderbird and Firefox belong in the same family, while still immediately recognizing that the new logo is the same Thunderbird you know and love.
It stuck so fast, I had to look app the current logo to remember how it looks like!