Consider this scenario: you are browsing a web site which at some point switches from http to https, and your browser show you a warning.
In Firefox you get:
Connection Partially Encrypted
Parts of the page you are viewing were not encrypted before being transmitted over
Information sent over the Internet without encryption can be seen by other people
while it is in transit
In Internet Explorer you get:
This page contains both secure and nonsecure items.
Do you want to display the nonsecure items?
Just an additional note: if you wish to control how your browser behaves in such situations, you can check (and change) the following settings in Internet Explorer and Firefox:
Quote of the day:
The release of atomic energy has not created a new problem. It has merely made more urgent the necessity of solving an existing one. - Albert Einstein