http://www.google.com/search?q=http
Microsoft is the first result.
Why Firefox but not IE? Firefox uses Google for any URL's which are invalid (so if you type in blah in the address bar it will take you to the first result for blah on google.)
[edit]IE would do something similar if you put in http by itself but using their search engine.
Anything Firefox doesn't recognise it uses the Google "I'm feeling lucky" to try and find it.
It has nothing to do with DNS or PTR records.
