Very true - it's definitely a symbiotic relationship. I doubt DDG could drop $300M a year, but MSFT could. Mozilla isn't going to give up that kind of $ just to hurt Google. The numbers vary by source, but across the board you see that FF's market share has dropped by about 1/3 or more since 2009, so every month that number gets lower (which seems to be the trend), FF becomes less and less valuable to Google.
Should have been clear. I wasn't saying beginning 2009, but their high in market share, which was late 2009. See http://en.wikipedia.org/wiki/Usage_share_of_web_browsers, which contains the graphic you linked to. In November 2009, FF's market share hit it's high of 32.2%. January 2013: 21.4%. Not FUD: the graph you linked shows the same. That's a reduction of 33%.