I've attached an add-on that tries to disable zooming using setZoomSettings, when the user clicks the browser action. It works in Chrome, and gives me this error in Firefox: Unsupported zoom settings: {"mode":"disabled","scope":null,"defaultZoomFactor":null} Looking at the code: http://searchfox.org/mozilla-central/source/browser/components/extensions/ext-tabs.js#996 it looks as if setZoomSettings only works if the new settings are the same as the existing ones.