NOTE: This article is no longer relevant for Firefox 56 or later.
Mozilla have decided to fix the CSS user preferences and the checkboxes and radio buttons are displayed correctly.
Remove the article updated settings (if applied) after updating Firefox.
Thanks to Linkesh for updating me on this issue.
NOTE: The following applies to Firefox 54 and later.
If the radio buttons and checkboxes exhibit collapsing behavior then comment out or remove the -moz-appearance: none !important; line located in the input[type="radio"]... section.
Thanks to ftobin for finding this.

The issue is that on some systems Firefox displays the