Google has long dominated the U.S. Internet search market for many years now--so much so that the company name has become synonymous with searching online.  However, last month Google's dominance slipped in the biggest drop seen since 2009, meanwhile Yahoo posted its largest gain.

Google's share of the search market fell to 75.2 percent in December from 79.3 percent a year ago, while Yahoo moved to 10.4 percent from 7.4 percent, according to the analytics firm StatCounter.  This shift put Google with its smallest share of the U.S. web search market since at 2008, when StatCounter first started tracking the numbers, and the highest share for Yahoo since 2009.

These changes in numbers were brought on by a deal in November between Yahoo and Mozilla, the makers of the Firefox web browser.  Yahoo replaced Google as the default search option for Firefox, a spot that had been occupied by Google since 2004. 

"The move by Mozilla has had a definite impact on U.S. search," StatCounter Chief Executive Officer Aodhan Cullen says. "The question now is whether Firefox users switch back to Google."

Firefox users represented a little more than 12 percent of the U.S. Internet usage in December, according to StatCounter.  This share gain is good news for Yahoo, as CEO Marissa Mayer pushes for more partnerships to boost the Sunnyvale, California based company's traffic and revenue. Yet it may be the biggest bump the Web portal gets, given that Firefox has been struggling to keep up with rivals, said Danny Sullivan, founding editor of Marketing Land & Search Engine Land. Google also has its Web browser called Chrome.

"I doubt Google needs to worry. For one, that's probably the high water mark," Sullivan says. "Unless Firefox suddenly grows share, everyone who likely could get switched has been now. And Google might claw back even the small share gone."

But Google hasn't disappeared from Firefox completely.  Users can still choose what search engine they prefer within the browser, and can always change their search engine preference at any time. 

The partnership between Yahoo and Mozilla that began in December is the start of a five-year deal between the two companies that makes Yahoo the default search engine for the Firefox browsers across both mobile and desktop.  Yahoo has worked to create a clean, modern and immersive search experience for Firefox users that will also soon be available to all users of Yahoo Search.