The option to disable JavaScript has been removed from Content tab of preferences dialog in Firefox 23, but you can still disable JavaScript through hidden preference in it. Firefox 23 is coming soon, now on beta channel and noticeable changes have been landed. Three options in Content tab- Block pop-up windows, Load images automatically and Enable JavaScript-and another one -“Always show the tab bar”- from Tabs tab have been removed from Preferences and they’re reset to defaults, that means as usual JavaScript is enabled by default.

Why should I disable JavaScript?

We’re not asking you to disable JavaScript for the sake of it, but for security reasons if you don’t want to allow JavaScript on webpages you view or for other reasons if you wish, you can disable it easily here is how.

Read: How to Disable JavaScript in Chrome, Firefox, Opera and Internet Explorer Browsers

firefox 22 with JavaScript option in Preferences

Disabling JavaScript from Firefox 23 onwards

1. Visit about:config in address bar

2. Find javascript.enabled preference and double-click on it to turn its value to false.

To re-enable it, follow above steps from 1 to 2 and change its value to true again.