Click-to-play in Firefox at least is a security feature. It's enabled automatically for known-insecure plugins like old versions of Java and Flash. You can enable it manually by setting a plugin to "Ask to activate" in the Firefox add-on manager:
https://blog.mozilla.org/security/2012/10/11/click-to-play-p...Click-to-play prevents Firefox from running any plugin code without explicit user action. I am 99% certain this is also the case for Chromium-based browsers. Source: I am a Firefox developer and I have worked on the click-to-play code, e.g. http://bugzil.la/899347
