Does this happen when you use the default style? Can you select it from the AdminCP and try it?
Using FF, I tried clicking on the Quick Nav Dropdown, both for the top breadcrumb and bottom one. Nothing pops open, and if you do both, your site locks up. I compared your JS versions, and code, against mine and they are both the same. You can try reuploading the JS directory (yourXenForoPackage/upload/js/), to be sure. Have you tried disabling additional JS that you may have installed, or placed in a template?