As far as I'm aware, I did not have a theme active.

I spent several hours debugging this. Chiefly the culprit turned out to be "Default Full Zoom Level" extension.

However, during the debugging process, I did a binary search through prefs.js looking for the problem. If I removed this line:

    user_pref("browser.startup.homepage_override.mstone", "29.0");

... the browser worked properly again. Very peculiar. Of course, this was written into the prefs.js again upon exit, so when restarted the browser was broken again.

Classic Theme Restorer probably wasn't the problem. Probably the problem only showed up because that was the first restart post-upgrade (to enable the extension).