From debian-user@lists.debian,org, Simon Piette:

http://www.privoxy.org/ is a good solution. It’s the following of
junkbuster. You can disable a plugin by default, and you can re-enable
them site by site. Since it will change the HTML code embedding the
flash file, so more messages about a missing plugin. Also, I think more recent
versions of mozilla have a switch for that, and maybe some prefs.js
editing could do it on 1.0.