Some sites prefer to use Flash but work perfectly well (or better) if Flash is not installed.

To apply this to the site of your choice: You can add the site's address to the user included sites for this script to blind them to your plugins. See screenshot below.

To test how this script should work: You can compare these pages:

https://www.jeffersonscher.com/res/plugins.html -- not blinded, sees all plugins
http://www.jeffersonscher.com/res/plugins.html -- script hides all plugins from the page

Inspired by this thread: https://support.mozilla.org/questions/1115681