Bundle the Thunderbird FireTray extension by default

There's a bug report on the paper cut project that says Thunderbird should simply minimize when closed instead of quitting, which is what it currently does. While this proposed behaviour is contrary to what we're trying to achieve with apps like Rhythmbox, in this case I believe it is appropriate for Thunderbird to behave this way since it is a communication app, similar to Empathy and Gwibber, both of which are allowed to run in the background, and are accessed via the messaging menu.

Thunderbird should behave in the same way, and according to the comment on the bug report, the FireTray plugin does just this.