Since Tails is a Linux distribution and the JavaScript exploit was specific to Windows (it worked by making calls to the Windows API), you are safe. Even if it worked on Linux, Tails *may* have prevented it with its transparent proxying, unless the exploit also rooted your Tails instance, which would be considerably harder than what it was designed to do. Everyone who recently switched to Tails was safe, which turned out to be a great move in this community, considering how many people used Tormail.