IE 11 isn't pretending to be Firefox - it's showing up the sites that treat IE as a second-class browser.
For years, web developers have complained that Internet Explorer didn't follow standards.
Starting in IE8, Microsoft made a huge effort to turn IE into a standards-compliant browser. With the notable exception of WebGL, a graphics standard that runs code directly on your graphics card which Microsoft has rejected for security worries, IE10 does a great job on standards that are actually part of HTML5 and mature enough to not need developers to keep rewriting their sites. It doesn't have all the experimental prefixes that testing sites that focus on WebKit give points for (but then neither do many of the websites you actually use).
Read more at source:
Windows Blue: Why IE 11 is taking a leaf from BlackBerry's book | ZDNet
