Why "Mozilla" string is present on all browser's User Agent?

Question

If you look on IE user agents you will see Mosilla string as well. Who knows why?

Answer 1

I think it had to do with: http://en.wikipedia.org/wiki/User_agent#User_agent_spoofing

Back in the early days, Netscape (code named Mozilla) was the main rival of IE, and they wanted to prevent being rejected by sites rejecting browsers other than Netscape.

Answer 2

Because that's how Netscape identified itself, and Microsoft wanted to work with sites that would detect Netscape and reject anything else.

Answer 3

It is a long and sad story.