If you needed any more evidence that the search wars are back on, just take a look at Google's tweet Wednesday morning, which shows Firefox users how to change their default search engine from Yahoo back to Google. The instructional tweet comes just one day after a new report showed that Google and Yahoo swapped 1.6 percent of market share last month.

Google saw its share of the U.S. desktop search space fall from 67 percent to 65.4 percent while Yahoo's slice of the pie grew from 10.2 percent to 11.8 percent from November to December, according to comScore. The data reflects the immediate impact of Mozilla's decision to end its 10-year relationship with Google and make Yahoo its default search engine for the next five years.

The 1.6 percent shift may seem like a small number, but it's a big deal in a $50 billion U.S. search ad market. Normally the search market sees changes of about 0.1 to 0.3 percent, if any change at all. The December number is also only representative of eager users who immediately updated to Firefox 34 -- the first version of the browser to ditch Google. As more users update their browsers, Yahoo's market share could increase further, and that's why Google felt the need to teach Firefox users how to adjust their search engine settings.

This could also just be the start of Google's troubles. Google remains the default search engine for Apple's Safari browser, but there are rumblings that Apple may also decide to quit its longtime rival for Yahoo or Microsoft Bing, sending the entire search market into full-out war.

For now, Google has to hope that Firefox users don't take a liking to Yahoo and decide to switch back.

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