This did not work for me. In fact classic theme restorer broke FF's Customize... menu feature, as well as the hamburger menu at the end of the toolbar - it is now completely inoperable, even after removing it.
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");
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).