How to disable JavaScript in Firefox

Up until Firefox 22, the current stable version of the web browser, JavaScript could be turned off in the content tab of the browser's options.  Here you only had to uncheck "enable JavaScript" to do so. A fast way to get there is to tap on the Alt-key, and select Tools > Options to do so.
With Firefox 23 comes a change that removes that option from the browser. It does not remove the ability to turn off JavaScript, only the menu option where it was listed in previously.
Whenever Mozilla makes a change to the browser, I'm trying to figure out why it was done. In regards to JavaScript, it appears to have been done so that inexperienced users do not turn the feature off by accident.

Firefox 22 Content menu

Firefox 23 Content menu

As you can see if you compare the two screenshots, both the option to turn off JavaScript and to block images from loading automatically have been removed from the tab.

Turn off JavaScript in Firefox

You still have a variety of options at your disposal to turn off JavaScript in the browser. I'd like to provide you with a selection of options:

1. Install the QuickJS extension for Firefox. It adds a single button to a toolbar that you can click on to turn JavaScript on or off. It also makes available the shortcut Ctrl-Q to do the same.
2. You can turn off JavaScript in about:config. Type about:config in Firefox's address bar and hit the enter key. Confirm you will be careful, and search for javascript.enabled there. Double-click the preference to change its value.
3. From Firefox 24 on, web developers have the option to turn off JavaScript temporarily. Open the Web Console via the menu or the Ctrl-Shift-K shortcut and click on the preferences icon there. Here you find disable JavaScript under advanced settings. This disables it only for the current session though.
4. Install the powerful NoScript Security Suite to turn JavaScript off for all sites by default, but with options to turn it on temporarily or permanently for select websites.

Side-effects of the change
If you have turned off JavaScript in Firefox currently and upgrade to Firefox 23 or newer, you will notice that it will automatically be turned on again without you being notified about it.
You are probably wondering why that decision was made, as it changes a user's custom configuration of the browser without notification. It is again designed for inexperienced users who have turned off the feature by accident. The reasoning here is that inexperienced users would have a hard time figuring out how to turn JavaScript back on if they made the change by accident, while experienced users will certainly be able to figure out what happened.
The advanced options of the dialog seem to be gone. I'm not entirely sure if Mozilla has added them as preferences in about:config as well. If the organization did, they do not seem to appear with a javascript tag in there.

Closing Words

I do not think I have ever used the preference to turn off JavaScript which I can attribute 100% to the use of NoScript. I do know some users on the other hand who browse the web with JavaScript turned off by choice, and they may have a problem with Mozilla turning on JavaScript automatically in their browser when they update to Firefox 23.