Just two things:
1: Only the last few versions of Flash will not be blocked, older and more insecure (Flash is never secure) versions will also need to be clicked.
2: Plugins are less useful nowadays, but still not useless: DRM protected content cannot be distributed or served one the net with the alternatives.
And here is a link for the aforementioned add-on: https://addons.mozilla.org/en-US/firefox/addon/click-to-play-per-element/