Google and Mozilla are no great friends of Microsoft. Yet Firefox is sending (via Google) tons of "innocent" visitors to Microsoft website for a simple reason - the user typed a bad web URL in the Firefox address bar.
Acutally, when Firefox is given an invalid URL, it doesn't attempt to determine the actual URL. Instead, it performs a "I'm feeling lucky" google search when confused and returns the first matching web page (see graphic).
So, whenever a Firefox or Mozilla browser user types an an invalid (malformed) URL into the Firefox address bar (like http//www.google.com - no colon) - Google redirects him to the Microsoft homepage as the top result for the search term http on Google is Microsoft Corporation.
Similarly, if the user types a secure malformed URL (starting with https//), he is redirected to the Google Adwords page as that's the first result for https on Google.
Source: Why do Malformed URLs Redirect to Microsoft?