Try checking your addons, any userscripts you may be using, your adblock, and your custom CSS. If you have ruled out these, I would try updating your browser and see if you still have the issue. Alternately you could let us know what version of Firefox you're on and I can try to diagnose the issue. If it's a very outdated version though we may not be able to guarantee support for it.

We should probably look into a more intuitive way to do this. But it's not impossible right now. You can just move the selected item to the 2nd spot, and then move the first item after it.