Oooh time for some history. Back in the days, the first basically usable Web Browser was Netscape Navigator. The Netscape User Agent already started with Mozilla (the internal codename for the program). IE followed later and added to it's user agent that it is compatible to the Mozilla agent. Later from the ashes of the Netscape Navigator rose the Mozilla project and Firebird (later: Firefox).
So with that user agent, you are masking your browser as IE 9.0.
Basically all of the big browsers still use the Mozilla part in their User Agent.