A Search Engine Watch Forums thread has a member stating he noticed Google began testing moving the AdWords ads (search ads) nearer to the free/organic listings. It is hard to describe, so I created a video below.
I illustrated how in Firefox, if you expand the width of your browser, Google will stop moving the ads to the right. However, in Safari, you can see how Google handled it in the past, by keeping the ads right aligned and shifting them over. Here is the video that explains it:
The first browser is Firefox, showing how it is more left aligned. The second browser is Safari, showing how they right align the ads.
Forum discussion at Search Engine Watch Forums.

Comments:
Jeff
08/06/2009 12:55 pm
Interesting. What's the result on IE?
simon
08/06/2009 04:03 pm
is it possible that this is more of a difference in rendering techniques between WebKit and Mozilla? Or is Google actually using different coding based on browser type?
duggan
10/19/2009 05:40 pm
i think it is the reason for high competition