1) Visit facebook.com
2) Type control-X.  Note that "C-x" appears in the minibuffer/echo area.
3) Type a sequence of j's and k's.  The screen scrolls down to the next item when j is pressed, and it
scrolls up to the previous item when k is pressed.  The "C-x" remains in the echo area, waiting for the
next key in the sequence that began with control-X.

Another site I visit frequently, inoreader.com, exhibits a similar problem.

In particular, this behavior means that I cannot invoke the kill-buffer command using its default
keybinding (C-x k) when viewing either of these sites.

Not all sites manifest this issue.  When visiting mail.google.com, I can invoke kill-buffer using C-x k,
even though that page also assigns a function to the k key (it moves the marker to the previous message).

The issue occurs even if I totally remove my ~/.conkerorrc.

My noticing the issue seemed to coincide with my recent update of xulrunner from version 19.0b6 to
version 27.0.1.