What does " Mozilla/5.0" in user agent string signify?

http browser user-agent mozilla
Random Guy picture Random Guy · Sep 5, 2012 · Viewed 14.2k times · Source

When I myself send many requests to the server I found it amazing that in IE if I choose opera user string that the value of user string was

User-Agent  Opera/9.80 (Windows NT 6.1; U; en) Presto/2.2.15 Version/10.00

But if I choose another browser in Internet Explorer that it puts Mozilla 5.0 in the user string first.

When I send the ajax request from Chrome that I found same thing that they put user string 

Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.20 (KHTML, like Gecko) Chrome/11.0.672.2 Safari/534.20

I found that Mozilla is an organization that doesn't have anything to do with Google and Microsoft. Perhaps it was a competitor for both. Why do MSFT and Google both put Mozilla in their user agent? Is there any reason for putting Mozilla in connection string?

Why do chrome and IE both put Mozilla in the userstring when they send the request? I do not know why but is there any specific reason for that?

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