Quote from tps2
Quote from Dr. Love
And on a different note..
Nobody still hasn't answered my question about internet browsers.. >.<..
I think there is not much diference in the code, more in the interface, but there are some incompatibility between firefox and IE, like the site Terra works better on IE and sometimes does not works at all on firefox and a friend of mine made a site using firefox to test, it worked perfectly, but on IE didn't work.
From my experiences in programming, the interpretation of the websites are similar, but probably not exactly the same. This is why sometimes a website looks different in one web browser than another. But, the most prominent differences is as tps2 said, which is the other stuff.
A lot of professional programs revolve around features and security. The features are usually things you can see for yourself, but for security, different browsers handle things such as downloading files and allowing access of a website to a person's PC differently. They also have different ways of filtering content and such. It really depends on how many possible vulnerabilities the programmers can think of, how they choose to handle it and how many bugs are generated and not caught.