Why does middle-clicking empty space go back a page?

This problem cropped up around version 5.0 and has persisted. Basically: when I middle click (often accidentally) on an area that doesn't contain a link, Firefox goes ''foreward'' to the previous page viewed in that tab. What I mean by "foreward" is that it is treated like a link navigation to the previous page even when there is no link to click and using the back button returns me to the page I came from. It is worth noting that this problem will also cause Firefox to return to pages viewed under "Private Browsing" mode even after returning to normal mode which is, at worst, a security risk. Is there an option that needs to change for this to stop happening? If there isn't I would like to file a bug.