What do you mean by disable the bars? They would be displayed but inoperable? That doesn't sound like something that could be accomplished without modifying the browser or installing an extension.

You could just not display those toolbars.

Type about:customizing<enter> in the address bar. Look for Show / Hide Toolbars at the bottom left.

There is also a Restore Defaults button at the bottom middle.

Note: about:customizing is no longer present in Firefox 47+ releases.

• Press Alt or F10 to bring up the toolbar.
  Then View > Toolbars > Customize.
• Right-click on a blank spot in the toolbar and select Customize.

See also:

• mKiosk: Public Firefox Kiosk Multimedia: https://addons.mozilla.org/firefox/addon/mkiosk/