The Firefox right-click bug
I was having a bug with Firefox where right-clicking in some places would bring up two context menus, a sort of double-menu with one on top of the other, and with both of them working (but clicking somewhere else outside dismisses only one of the menus).
I searched around on Google a bit and found no mention of it… Recently, one of my friends had the same bug, so we knew what to do — look in the list of extensions that we had both installed, and find which one was causing it. The culprit turned out to be Image Zoom. I uninstalled it, and now everything works fine.