Firefox already ships some UA overrides. You can see the list of active overrides (with links to bug reports explaining each) in Firefox's about:compat page.
Well browsers with a different engine do work differently. By the way, I do disagree with this strategy. Just saying it is not necessarily something illegal.
Problem was in this case it seems the product works fine across different engines, they just decided it was ok to block a competing product to limit comsumer choices and generally make the web a worse place ;-)
Of course it doesn't have to be that black and white but they certainly makes it easy to dislike them at the moment.
