Google Earth 5.2 no longer uses the operating system's web browser when you click on links. Google Earth comes with a WebKit-based browser, so you'll never have to leave the application to open a Wikipedia page or the website of a local business.
"Sometimes when you want more information, you may want to click through to a link to see the full Google Places page for a business, or learn more about a photographer whose photo you really enjoy. In the past, this has required opening a link in an external browser to see the full page. For Google Earth 5.2, we've added an embedded browser that lets you browse the full web. Click on a link, and the browser pane slides across the screen. When you want to return to the Earth view, just click the Back button," explains Google.
As in the previous versions, Google Earth for Windows and Mac also includes a plug-in that lets you embed a Google Earth view in any web page. Google Maps is the most popular service that lets you use Google Earth in your browser. Now you can browse the Web in Google Earth and use Google Earth in a Web browser.
If you don't like the embedded browser, you can disable it by going to Tools > Options > General and checking "Show web results in external browser".