Basically the developers are loathe to create a menu item for a simple pref such as that, which usually needs to be turned on just once.

https://bugzilla.mozilla.org/buglist.cgi?quicksearch=plugins.click_to_play

Sorry, I didn't see that mentioned in the "click to play" Bugs list. If you think that feature is important, file a RFE (request for enhancement) Bug.

If you do decide to file a Bug report, read this first - to give your request the best chance of being approved.
https://developer.mozilla.org/en/Bug_writing_guidelines

Well I would say it's nice to have the feature, but it's a shame the general users don't have this option through the GUI through a simple checkbox, there's plenty of room in the 'Content'-tab of the Options dialog ..

I wouldn't doubt that an addon developer comes up with an extension to toggle that pref on and off - fairly quick.

Another option would be the Custom Buttons extension, to make a button for that feature.
https://addons.mozilla.org/en-US/firefox/addon/custom-buttons/

User created custom buttons are discussed here.
http://custombuttons.sourceforge.net/forum/viewforum.php?f=4&sid=e38e7f6b4ef490de16f2e1d1215e4adc

Sure, I know. But I think since this was one of the more awaited features that it would be natural to have it's own checkbox like the "Do-not-track" option.

See also:

• https://wiki.mozilla.org/Opt-in_activation_for_plugins
• bug 738698 - [meta] Users should have the ability to activate plugins on demand
• bug 711552 - Create click to play UI for desktop

Thank you very much cor-el.