From their Privacy Center page (http://www.google.com/privacy_ads.html):
"No personally identifiable information – We don’t collect or serve ads based on personal information without your permission."
Notice the last 3 words...yet you are by default, "Opted In"!!
This constitutes a serious disregard for "informed consent".
And Gmail, once perceived to be 'safe', actually scans all your mail & then selects advertising based on the content. It's "for your own good...."
http://mail.google.com/mail/help/about_privacy.html
"Google uses the DoubleClick DART cookie on our Google content network ..."
These guys are the used car salesman of the internet, very 'slippery' characters burying their methods, details & opt out choices with layer after layer, page after page of non-sense & merry-go-round tactics plus non-functioning links. You know: the usual corporate greed tactics.
A few tactics to stop this crap:
"Opt out" sign in pages;
Google here: http://www.google.com/privacy_ads.html
The following tips help, but methods (vectors) constantly change.
FIREFOX: Tools> Options> Privacy> top drop menu set to "Firefox will use custom settings..."> Check 'Accept Cookies from sites'
►& Un-Check 'Accept 3rd Party cookies'
My own choices (from Mozilla) to bolster Firefox is here:
https://addons.mozilla.org/en-US/firefox/collection/dunbarpappy
(At least get the #1 item for malware prevention: NoScript.)
^Most block automatic server traffic until you approve of the action.
They also speed up browser page loads, as they block a lot of the 3rd party assets being requested from remote servers.
►Special note: Firefox has been hacked by Microsoftie.
Microsoft has installed (unannounced to you) into Firefox, rubbish that degrades the security of Firefox to little more than IE.
Check your FF 'Add-on's'> "Extensions" & "Plug-in's" box and uninstall/disable NET framework, Windows Presentation Foundation, Shockwave, Silverlight, Java stuff, or other active scripting rubbish.
These hacks re-enable the JavaScript exploit avenue.
http://www.grc.com/sn/sn-199.htm