Google Inc. on Wednesday ran an ad for Firefox on its popular homepage, giving the open-source Web browser a big marketing boost over their joint rival Microsoft Corp.

The ad promoted the key advantage supporters claim Firefox has over Microsoft's Internet Explorer -- security. IE, which has more than 85 percent of the market, is a favorite target for hackers and virus writers.

The one-line text ad read, "Firefox with Google Toolbar: tabbed browsing, safer surfing," and included a picture of the toolbar on Firefox. Only people using IE could see the ad.

"We're super excited about it," Mary Colvig, spokeswoman for Mozilla Corp., said. Mozilla, the for-profit subsidiary of the Mozilla Foundation, develops and markets the open-source Firefox.

Mozilla declined to discuss the ad, referring all questions to Google. A spokeswoman for the Mountain View, Calif., company confirmed Google ran the ad.

"Google is promoting Firefox with Google Toolbar because tabbed browsing and safer surfing help provide our users with a great search experience," the spokeswoman said in an email.

Google has run promotions before of its products, bundled with third-party offerings, the spokeswoman said. For example, the company has run ads about Google Video that highlighted content from the CBS network and the National Basketball Association.

Goggle has had a joint promotional relationship with Mozilla since last year, when the search engine started providing text ads promoting Firefox plus the Google Toolbar to its advertising network of third-party Web sites.

The latest ad reflects Google's continuing close relationship with Mozilla. In his blog, Paul Kim, director of product marketing for Mozilla, said "thanks to the marketing team at Google for the support."

The normally ad-free Google homepage is known for its clutter-free interface and simplicity, which has always been an attraction for Web surfers in need of a search engine. In March, the page attracted 89.8 million unique visitors, according to Nielsen/NetRatings. The vast majority of people visiting Google go to its homepage. Its other domains, such as shopping, local search, etc., attracted a total of 3.1 million unique visitors in March.