Avoid hiding and keeping track of private windows

[keshto]

Avoids adding the firetray Handler to windows that are private. This makes it so that when a user is surfing privately and exits the window it is not kept. Which would allow other users of the same machine to see the users session, which defeats the purpose of a private session.

[foudfou]

Nice! Thank you. Sounds reasonable. Could you add a pref for that (can't think of an obvious name right now) ? See 0f28d23 for how to do it. You should also test under Thunderbird and Seamonkey, and think of other supported applications.