I was rather annoyed by this as I use Google a lot when pentesting and often like to copy & paste links from the search results. Instead of getting a direct link to a site, I end up with a huge google.com link. So I decided to write a little Greasemonkey script to fix this:
Another nice extension is Adaptive Referer Remover. This add-on allows you to remove Referer headers if they contain certain patterns. So I added the following pattern to prevent sites from seeing what Google query I entered to find them (you could add rules for other Google features as well or even block it entirely):
This add-on refuses to install if you use Firefox 3.5, but it works fine in that version. You can install the Nightly Tester Tools extension, which enables you to install it anyway (or you could edit the .xpi file manually).