Google-chrome – How does Google Chrome know the Firefox history if I never imported it

Sadly, this is normal. For whatever reason, the Chromium team has yet to implement a comparable addressbar/history feature to Firefox's.

In the meantime, you can try the Better Omnibox extension to improve history and bookmark support in the Omnibox.

If you pay attention to the status bar while hovering the link, you'll see that it initially points to the "clean" URL.

Only when you click on the link (with any of the three mouse buttons), a JavaScript event gets triggered that changes the link's destination to Google's URL redirection.

To verify my claim, just right-click any of the purple links and close the context menu. Unless you already visited the site from within Google's search results, you'll see that it changes colors.¹

I don't know exactly how Google injects the redirection URL², but the general idea is this:

// define a function `f' that changes a link's clean URL to a redirection URL
var f = function () {
    // prepend `http://www.google.com/url?rct=j&url=' to the link's target
    this.href = 'http://www.google.com/url?rct=j&url=' + escape(this.href);
    // don't invoke this function anymore when clicking the link
    this.removeEventListener('click', f);
    // don't invoke this function anymore when right-clicking the link
    this.removeEventListener('contextmenu', f);
}

// save all <a> tags in an array `a'
var a = document.getElementsByTagName('a');

// for each <a> tag in the array `a'
for (var i = 0; i < a.length; i++) {
    // execute function `f' when clicking the link
    a[i].addEventListener('click', f);
    // execute function `f' when right-clicking the link
    a[i].addEventListener('contextmenu', f);
}

You can try this jsFiddle to see how it works.

_{¹ Tested on Chromium 25 (Ubuntu 12.10) and Chrome 26 (Windows 7)}

_{² Minimized JavaScript is a bit difficult to read.}