This page describes the JavaScript Debugger as it appears before Firefox 52.
To pretty-print a file that has been minified, open the minified file and click the icon that contains a pair of braces:
The file will now appear in a more readable format.
You can instruct the debugger to detect minified sources and pretty-print them for you automatically, by selecting "Auto Prettify Minified Sources" in the Debugger settings.