By blocking IFRAMES with NoScript blocking turned ON (you have to turn forbidding IFRAMES on manually in options http://i.imgur.com/7jctoTW.png) I managed to block IFRAMES on google and w3school pages.
!!!Text in iframe "Test page in iframe" on quirksmode test page was still shown even after I have frames turned OFF in about:config and I block all scripts and frames and iframes with NoScript.
I didnt test Chrome at all.
Possible things that can cause bug:
•I am using NoScript 2.6.7, I turned it off and on but it is possible that it is overriding Firefox settings in about:config. when you serach about:config for "frames" there are many settings mentioning frames from NoScript and AdBlockPlus.
•AdBlock Plus 2.3.2? Same reason as NoScript.
•Fot the first time I noticed Shield in the address bar with "Firefox has blocked content that isnt secure" bubble. http://i.imgur.com/K4FL65n.png. I dont know how long this feature is implemented or what exactly it does, here are some details: https://support.mozilla.org/en-US/kb/how-does-content-isnt-secure-affect-my-safety?as=u&utm_source=i...
Re: Is turning off iFrames via about:config > browser.frames.enabled;false broken in Firefox 23?
I dont know if that feature was working in any time during development. I just found suggestions on google that this is the way to turn off iFrames in Firefox.
I didnt see anybody complaining it doesnt work, but also no conformations that is works.