Multi instance vs Single instance application

I was asked this question in an interview. Why does firefox show开发者_高级运维 a single process in the task manager even when multiple instances of the application is open while Internet Explorer 8 and Google Chrome show multiple processes in the task manager? So what factors decide this kind of behaviour in different applications? Please answer as soon as possible!

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There is always only 1 instance of firefox open per session, but it can have multiple windows. Thats different from chrome and IE where for each tab and window, a new instance is created. Both have advantages and disadvantages.

For starters, if in IE or Chrome, 1 page crashes (due to plugins or bugs or whatever), only that instance crashes and that wont affect other instances. Its also more secure, hacking 1 page doesnt mean you have access to the whole browser since all pages are isolated. The cost is a little loss in performance since you have the overhead of windows for each instance and no easy way of sharing resources.

EDIT

Please read: http://blog.chromium.org/2008/09/multi-process-architecture.html