With Firefox 40+ adding

add_hook("mode_line_hook", mode_line_adder(downloads_status_widget));

breaks all keybindings.

This line below from the output looks suspicious.

354:Console error: [JavaScript Error: "NS_ERROR_UNEXPECTED: Component returned failure code: 0x8000ffff
(NS_ERROR_UNEXPECTED) [nsIDownloadManager.activeDownloadCount]" {file:
"chrome://conkeror/content/mode-line.js" line: 322}]