Firefox is only a few years old, yet there isn't a single site I've visited in recent memory that doesn't work properly. For a browser that "only" has ~25% of the market, every single site out of thousands I've visited not only work properly, they tend to break in IE 6/7 and not standards-based browsers.
There has been a mass of work done on the developer side of things across thousands of public-facing web properties in the last few years to be much more standards compliant.
Microsoft shouldn't worry about "breaking the internet". They've already done that. By just releasing updates to their browsers on a regular basis, orienting around w3c standards, and providing definitive support cut-off dates for old software, they will easily move the mass of their users forward.
Its very simple, really. If they simply just adopt web standards and stop supporting their old crap, suppose it breaks some Fortune 500 internal sites. What are these companies going to do, drop Microsoft and move to other standards-based solutions? In the end, they end up using modern software anyway. Microsoft is creating their own problems.