"Auto Prettify Minified Sources" setting of Firefox Firefox Developer Tools turns the Prettify Source feature on by default.
To enable it:
- open Firefox Developer Tools (Ctrl + Shift + I)
- Debugger tab
- engine icon on top right of the Debugger tab, not the global settings engine
This is documented at: https://developer.mozilla.org/en-US/docs/Tools/Debugger/How_to/Pretty-print_a_minified_file
Tested on Firefox 42.