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I think that this problem was caused by Firefox's caching
of an older help_controls.js: at some point it suddenly
started to work, perhaps after I had forced a reload
somewhere.

I went back to an older version of my ~/.mozilla 
directory and the problem was back. I couldn't
figure out how I had fixed it before, so I deleted
the whole ~/.mozilla/firefox/default.cdh/Cache
and now it works.