Several people have pointed to Google advertising Firefox with Google Toolbar to some users. I guess this can be seen as good news for Firefox as it could mean Firefox gets out to a larger audience.
However, I don't really like the fact Google is effectively trying to become the main and primary distributor of Firefox. It's Adsense program has encouraged many users to link to Google's Firefox download page which comes bundled with the Google Toolbar rather than linking to Mozilla's official page.
Google's advert seems to be more of an advert for Google Toolbar than Firefox. Now perhaps thats the only way Google can justify advertising Firefox and it's better than them advertising Google Toolbar for Internet Explorer because it still gets people to switch to Firefox. However, if everyone downloads Firefox through Google, people begin to think Google Toolbar (which some people may consider to be spyware/bundleware) is an essential part of Firefox.
For example Dave Winer, who invented XML-RPC, RSS 2.0, OPML, etc., got confused and thought that there was no choice but to download Google Toolbar with Firefox.
Another issue is people loading their webpages chock full of Firefox adverts and notices to make money on Adsense. This can give users a negative impression of Firefox.