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}]

Oddly this bug manifests only in firefox -app and not in xulrunner.

I was able to reproduce the bug in firefox 40.0.2 but not in xulrunner 40.0.2 and another user reported
the widget working fine in xulrunner 40.0.3.