How to make sure that a Firefox plugin never activates (again)

"The web is moving away from plugins which is a good thing as it reduces the risk of falling pray to security attacks that target old versions of plugins or use 0-days to attack even the latest versions. Some plugins do cling on however despite the move to HTML5 and JavaScript. Especially Java and Flash need to be mentioned here, but chance is that your browser picked up quite a few plugins along the way.
"Firefox is still picking up plugins that get installed on the system automatically which can be quite problematic if you do not want the browser to do so. ... "
Source: Ghacks Technology Newsletter
Details  How to make sure that a Firefox plugin never activates (again)