Webconverger has its own Firefox kiosk extension to tweak the user interface elements such as the URL bar and icons.

Github: https://github.com/Webconverger/webconverger-addon/

Default

chrome=webconverger

The goal with this default to have a stable, simple & familiar user interface for using the Web.

webconverger

showprintbutton API is enabled on this screenshot. Normally there would be no print icon.

No address bar

chrome=webcnoaddressbar

Customization option, perfect for filtered setups. For customers who want patrons to browse, but not enter in URLs to navigate away.

webcnoaddressbar

showprintbutton API is enabled on this screenshot. Normally there would be no print icon.

Fullscreen

chrome=webcfullscreen

Customization option. For customers who want to do basic Digital signage or just show off some pictures somewhere.

Neon has further tweaks for the digital signage use case, such as a black background.

webcfullscreen

If you're concerned about the scroll bar, alter the overflow CSS element.

Source code

Preferences locations

Locking down Firefox is a project on its own. In Webconverger logic to disable various Firefox anti-features reside in:

Notes on keeping the extension free of cruft

Look for ids upon the browser.xul that introduce new unwanted UI elements. Then block them in the https://github.com/Webconverger/webconverger-addon/blob/master/src/webconverger.css.