Firefox will load whatever URL is in the copy-paste buffer in the page if you middle click somewhere on a page. My mouse’s wheel also serves as the middle button, and I found it really annoying that when occasionally middle-clicking while scrolling the page, Firefox would try to load another page …
You can control this behaviour with this about:config property middlemouse.contentLoadURL. Set to true, the middle-click on the page will load the URL (if it is one) in the page, set to false it won’t.
Thanks Ayman !