If you can reproduce this problem in a clean profile, please file a bug, cc "bzbarsky", and I'll take a look. Obviously, steps to reproduce would he helpful too.
If it doesn't happen in a clean profile, then we start looking into what's special about your existing profile.
I'm not sure what I've done to make it not work here, but this profile dates from a long time ago so maybe it's just cruft or an extension behaving oddly. I'll switch to using a clean profile. Sorry again and thanks for your attention.
If you still have the profile where it doesn't work, and are willing to do some detective work to figure out what it is that doesn't make it work (e.g. extensions or prefs.js or something else), that would be awesome. You're hardly the only Firefox user with a somewhat old profile, and it's possible that there's something specific going wrong that we should be handling better on our end...