Seconded. Google does this too; if you use gmail or google groups and don't log out you'll find that you are "recognized" at search pages, comment posting forms at some blogs, and elsewhere on the net. That's where it visibly seems to know who you are. Given the nature of Google's business I'll bet your searches, blog participation, and other non-gmail non-gg activities are being tracked in some huge database they mine, probably for targeting AdWords.

Of course, it stands to reason that you're recognized and may get creepily targeted advertising whenever you later encounter an AdWords block on a random Web site while logged in to gmail.

More than that, though, there's Google Analytics. If you know what you're doing you can check if a given site uses it, because if it does it will load a script called "urchin.js" from a Google Analytics domain. The potential for tracking between a perpetual gmail logon and Google Analytics covers an even larger subset of the web than AdWords blocks -- perhaps half of it(!)...

Of course you can log out. Google-affiliated pages that actually explicitly recognize your gmail id offer a logout link (and a login link when you're logged out). This includes Google itself, SERPs, those blog comment pages, and of course gmail itself and google groups.

Firefox provides more potent defenses against this type of tracking. Aside from cookie munching options, you can install two useful extensions: Adblock and Tab Permissions. The former lets you block, in fact, any loadable page elements, not just images and Flash. Images of course can be blocked with a couple of mouse clicks on the offending animated .gif. Flash can be done via the Adblock menu accessed by right clicking "Adblock" in the lower right corner of the browser tab, and "Overlay Flash", then click anything now showing "Flashblock" that used to show something animated, annoying, and advertising some product you'll never buy. But you can block anything by left-clicking that "Adblock" if you know what you're doing. Sorting the table that appears by URL helps, as everything "native" to the site you're at will clump, as will everything native to doubleclick.net, Google, and so forth. This makes locating dubious off-site materials, even 1x1px Web bugs and questionable tiny iframes, easy -- and killing them, too. Best of all, you can nail any Javascript loaded by external file. Iframe killing lets you nail AdWords blocks (and can also, I think, be accomplished by right clicking in the iframe.) Javascript you can block includes urchin.js, the Google Analytics tracking bug. So much for your every step on the Web being tied to your gmail account then. As an extra added bonus feature, the scripts responsible for "Intellitxt" and similar-style advertising, which turns words in the editorial content of a page into links to irrelevant stuff (read: advertising partners) and is seriously unethical (blurs the line between advertising and content!) can also be nailed. Try just inserting a block rule for "intellitxt.js" to get rid of Intellitxt; reload a page that had the bogus links and it will be refreshingly clean and readable and the only links you'll see will be genuine links that were actually inserted by the author of the content. Nifty!

Where Tab Permissions comes in is with the small but growing set of sites that try to block Firefox+Adblock users. The examples I've seen so far rely on Javashit to pull off their tricks. Installing Tab Permissions initially seems not to alter your Firefox. You need to customize a toolbar first -- there are now new icons for things like "disable meta redirections" and "disable Javascript" that you can add to a toolbar. Those are the two you want, by the way; I put mine between the address bar and the google search box. If you hit a site that says it's blocking you because you block ads, disable JS and reload. If you get redirected to a page that just says "This site requires Javascript", as is all too likely, disable meta redirects and hit back, and if you end up back 2 steps, hit the link you originally followed. If you still see "This site requires Javascript" or the site is now simply blank, mangled, or unnavigable, then you're probably going to have to either skip it or resort to IE (ugh), no thanks to their evil use of scripts presumably to render all of the content that should just be HTML. :P But I've run into very few of those so far. (You can also try disabling Adblock for the one site and see if it works then. It's also possible that they use server-side methods to exclude all Firefox users. If they do something that evil, you'd best just ignore them, but if you really must use their evil, horrid site, you can also find tools to spoof the browser's self-identification as Internet Exploder or whatever else.)

With a bit of experience and those extensions, a Firefox user should be able to greatly increase their surfing privacy; only the sites you visited will directly know that you visited them, and all they'll have is your IP address, which usually means the most they know is your city and your choice of internet provider (assuming you even had one). Third parties like Google will know nothing unless the site shares its server logs around or something.