This app places a couple files inside the Firefox application that tell Firefox how to draw the widgets that you find on websites. These include text buttons ("Submit," "Cancel," etc.), text input fields, radio buttons, checkboxes, pull-down lists, etc. Drag it on a second time to revert to the default widgets. Or just open the app and it will prompt you to locate Firefox.
The screenshot shows what you buttons and "widgets" look like after you use Firefoxy.