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!

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