User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:37.0) Gecko/20100101 Firefox/37.0 Build ID: 20150404092546 Steps to reproduce: Visit a website containing a "Browse" button for uploading files. Click on the button. Sites tried include a webmail site and amazon.ca. Additional notes: Using Firefox 37.0.1 on Arch Linux with XFCE. Firefox can successfully open a file manager window to select a download location. Actual results: Nothing for the user. The Browser Console indicates "TypeError: ev.popupWindowURI is null", each time the button is clicked. Expected results: A file picker window should open to allow a file to be selected.