Google hits Microsoft where it hurts -- the browser

Google's (GOOG) Chrome browser has overtaken Microsoft (MSFT) Internet Explorer as the most popular Web interface in the U.S.

A study by software maker Adobe Systems shows Chrome with 31.8 percent of the browser market, compared with 30.9 percent for IE and 25 percent for Apple's (AAPL) Safari. Mozilla Firefox's share of the market dropped to 8.7 percent, from almost 20 percent just two years ago.

The shift underscores the importance of free software in a continuing three-way battle between Google, Microsoft and Apple for control of how consumers interact with the Internet. Both Google and Apple enjoy the advantage of large mobile presences, which allows them to promote their own browsers. In addition, Google has gained significant ground on PCs.

Although Microsoft long held a home-field advantage with its domination in PC operating systems, the company's Windows Phone platform has a tiny market share for mobile browsers. Usage of Firefox's mobile platform depends on consumers downloading and installing it on their devices as an app. People want a "consistent experience across devices," according to Adobe Digital Index, a site that covers digital marketing trends.

Browsers have long been available for free download, although apps are increasingly replacing them on smartphones, tablets and other mobile devices. But browsers remain important because, like other free software and services, they are a proxy for consumers. If people choose one vendor's browser, then the company can turn it into a platform and open it to developers, as Google has done with Chrome. The hope is that the more accustomed people are to a company's browser, the more likely they will be to use other products and services that the platform maker and its allies provide.

Microsoft long used IE as a weapon in corporate computing. Many companies built internal Web-based applications optimized for the software giant's browser. But the growing popularity of Chrome could persuade some companies to change their software to work with Chrome as the front end.

Search engines are another front in the battle among big technology companies to corral users. Apple has made Microsoft's Bing the default search engine for the Siri voice interface in its iOS mobile platform. Apple could also potentially turn Bing into the default way to search. Search ad revenue is a vital source of revenue for Google, and any significant inroads by Apple in search could threaten the the search giant's business.