Password protecting firefox / IE

Hey all.
Due to my computer often being left on at home, usually due to a download in progress, I would like to be able to set up my computer so that a password is needed in order to open / use firefox and IE.
I know that there is a way of doing this in IE but it means that a password is needed everytime you change web page, which frankly is a pain. So im asking if its possible to do this just for when you open the browser. Yet on the otherhand does not restrict internet connection so that said download can continue.
Thus meaning i can leave my download in progress and be safe in the knowledge my older sister cant get onto my computer and my younger sister cant access unsuitable sites.
Any ideas?
Mass
Due to my computer often being left on at home, usually due to a download in progress, I would like to be able to set up my computer so that a password is needed in order to open / use firefox and IE.
I know that there is a way of doing this in IE but it means that a password is needed everytime you change web page, which frankly is a pain. So im asking if its possible to do this just for when you open the browser. Yet on the otherhand does not restrict internet connection so that said download can continue.
Thus meaning i can leave my download in progress and be safe in the knowledge my older sister cant get onto my computer and my younger sister cant access unsuitable sites.
Any ideas?
Mass