Firefox will even go as far as to reenable Javascript the following day should a user disable it. More advanced users can force change their Javascript preferences back to the old approach by manipulating about:config. This method is not meant for the average user.
One of the pluses of this new approach will be to help less sophisticated web users who accidentally turn off Javascript and are frustrated "when the web doesn't work." Or perhaps they were trying to turn off Java and accidentally disable Javascript. The concern which caused this new update for Firefox 23 is that some, simple checkboxes can "wreck the user experience" and cause tremendous confusion for many web users. Javascript is one of those core web design tools that are seemingly mandatory for modern web applications. Some in the web development community are frustrated by this lack of control over Javascript preferences. There is concern that leaving Javascript permanently on (and without ability to disable it at will) is not "universally safe."
SOURCE: https://bugzilla.mozilla.org/show_bug.cgi?id=873709&resub