Add Firefox buttons to manipulate about:config settings

gistfile1.js

/*

I needed a button in the Firefox UI to toggle the default page zoom level 
setting offered by the NoSquint extension (shown as "extensions.nosquint.fullZoomLevel" 
in about:config). Since this isn't likely to be provided directly and I 
wasn't interested in writing my own extension for something so simple, I 
looked for an extension that would expose about:config settings to buttons.

Instead, I found the Custom Buttons extension:

https://addons.mozilla.org/en-US/firefox/addon/custom-buttons/

Custom Buttons lets you put some javascript behind a button that can be 
placed on your toolbars. The code operates as a XUL overlay, so you have
access to Firefox's internal APIs. Here's the code I used:

*/

var prefManager = Components.classes["@mozilla.org/preferences-service;1"]
                    .getService(Components.interfaces.nsIPrefBranch);
                    
// Available methods: getIntPref, getBoolPref, getCharPref
var currentZoom = prefManager.getIntPref( "extensions.nosquint.fullZoomLevel" );

if ( currentZoom < 150 ) {
    currentZoom = 150;
} else {
    currentZoom = 100;
}

// Available methods: setIntPref, setBoolPref, setCharPref available
prefManager.setIntPref( "extensions.nosquint.fullZoomLevel", currentZoom );

Thanks so much for this. I spent about 30 minutes trying to figure out how to change prefs from JS to do this exact same thing.