The problem is some uses of flash doesn't put any flash controls on the webpage. Mainly for audio related uses, like audio players (mixcloud), or games (since html5 audio still has some work to be done). Chrome gets around this with an icon in the urlbar, and an option "Run all plug-ins this time". If firefox wants to make click-to-play the default option, I don't think this is going to cut it. The only two options they have are to only click-to-play visible plugins (but this is really hard to detect), or give a popup. The popup might work for most users, but some are just going to get confused. In fact, I don't know if most users will read the place-holder for click-to-play in general, or just go "why isn't youtube working".
There's an “enable plugins” icon at the left of the location bar when the page contains those. It's a bit less visible than the one for pop-ups, but at least it's in an area that can't be spoofed.
