madalu: Thanks for these hints. Unfortunately they're not enough hints
for me to actually find a fix.

At least the following patch does _not_ suffice:

--- a/modules/window.js
+++ b/modules/window.js
@@ -198,6 +198,8 @@ function window_initialize (window) {
     // Add a getBrowser() and content to help certain extensions designed
     // for Firefox work with conkeror
     window.getBrowser = window_get_this_browser;
+    window.gBrowser = window_get_this_browser;
+    window.getBrowserForContentWindow = window_get_this_browser;
     window.__defineGetter__('content',
                             function () {
                                 return this.buffers.current.browser.contentWindow;