Update: It seems this “feature” is being disabled by default as of Firefox version 57, due to it now being recognized as a bug (unwanted behaviour) for Linux versions of the browser.
One little annoyance I’ve had recently is accidentally middle-clicking on a web-page when my clipboard contains a URL. When this happens, it load that page on the current page. I don’t find this behaviour particularly useful, and more of a frustration. I originally thought it was due to an add-on, but I discovered is a built-in setting in Firefox, so I disabled it. In the about:config menu, set the following to false: