In Mozilla/Firefox for Linux, if you middle-click in a browser window, the contents of your clipboard are sent to Google, and to whatever web site shows up as the first search result with your clipboard contents as the query string.
This is a feature. In Unix-like environments, selecting something copies it to the clipboard, and middle-click pastes—and this is often very handy. The Linux version of Firefox (and Firefox for any other platform with this feature explicitly enabled) allows you to middle-click-paste any URL into any browser window to visit the URL. If what you paste is not a URL, it treats it as a Google search, with Google's "I'm Feeling Lucky" feature turned on, so you automatically go to the first search result. That web site gets the Google search query in the referral string.
I almost never use this feature on purpose. But I almost always click on links with the middle mouse button, which, on a link, opens the link in a new tab. If I slip and miss the link when I click, whatever is on my clipboard—perhaps something I copied a long, long time ago, perhaps something personal or confidential, perhaps something I selected by accident while dragging my mouse around—becomes a Google search and ends up in somebody's referral log. This happens so often, I have to consider it a security risk.
To disable or enable this feature in Firefox: Enter about:config in the address bar, then find the preference called middlemouse.contentLoadURL. Double-click on it to toggle its value: "true" means it is enabled, "false" means it is disabled. Notice that middle-click for opening links in tabs, and middle-click for pasting into form fields or the address bar, are not affected, which is exactly what I want.
Wow, what a misfeature. I can see middle-clicking in the location bar doing that, but in the page? Duh.