As you may know, Yahoo recently became the default search engine in Mozilla’s Firefox browser after Google had held the spot for a decade. As Yahoo’s search market share has already benefited from the switch, Google is telling Firefox users to switch back.

On Wednesday, Google tweeted this helpful little video demonstrating how to change the default search experience, in case users who care enough about Google to follow the company on Twitter couldn’t figure out how to do that.

Danny Sullivan points out that Google is now telling Firefox users who visit its homepage to set the default experience back to Google as well. It displays a message that says, “Get to Google faster. Make Google your default search engine.”

Yahoo itself has been telling visitors to its homepage to “upgrade” to the new Firefox:

A couple weeks ago, StatCounter released some data on search market share in the U.S. finding that Yahoo saw its highest amount of that in over five years in December, thanks to its new Mozilla partnership.

“The move by Mozilla has had a definite impact on US search,” said StatCounter CEO Aodhan Cullen. “The question now is whether Firefox users switch back to Google.”

It will be interesting to see January’s data. Google is obviously worried enough about it to tweet out explanations on how to switch back.

This week, comScore also put out its monthly look at U.S. desktop search engine rankings for December. From that:

Google Sites led the U.S. explicit core search market in December with 65.4 percent market share, followed by Microsoft Sites with 19.7 percent (up 0.1 percentage points) and Yahoo Sites with 11.8 percent (up 1.6 percentage points). Ask Network accounted for 2 percent of explicit core searches, followed by AOL, Inc. with 1.2 percent.

18.7 billion explicit core searches were conducted in December, with Google Sites ranking first with 12.2 billion (up 2 percent). Microsoft Sites ranked second with 3.7 billion searches (up 5 percent), followed by Yahoo Sites with 2.2 billion (up 21 percent), Ask Network with 372 million (up 5 percent) and AOL, Inc. with 222 million.

As its share rises, Yahoo is also testing out a search results page layout that more closely resembles Google’s:

You’d have to think a similar look and feel to Google could keep some used to the Google experience from Firefox from bothering to switch back compared to a more drastic change such as Yahoo’s current layout.

Apparently Bing’s actually testing a similar look as well.

This might not be a good time for Google to be losing any search market share, considering that analysts have already grown concerned by slowing growth in its core ad business.

Images via Mozilla, Google, Yahoo, StatCounter, comScore