I think using an add-on you can present an empty navigator.plugins collection, which is the browser information many sites use to check for Flash. That's the first method used on my test page here:

https://www.jeffersonscher.com/res/plugins.html

It would be more difficult to just strip out Flash and show the rest of the list, and that level of complexity might not be necessary if it's just for a few particular problem sites that don't use any other plugins.

I haven't finished my research, but plan to look into it after hours. Hopefully I can do it with a Greasemonkey user script (since actually doing a full extension is beyond my abilities).

Okay, here's one way, using the Greasemonkey extension and a user script.

(1) Install the Greasemonkey extension from the Add-ons site:

https://addons.mozilla.org/firefox/addon/greasemonkey/

(2) Install the "Hide All Plugins" user script from Greasy Fork:

https://greasyfork.org/scripts/18256-hide-all-plugins

It's already set up for http://video.gazzetta.it/

See also:

Bug 1169945 - Remove unused plugins.enumerable_names whitelist that hides plugins from navigator.plugins enumeration Bug 1186948 - remove plugins that are click-to-play from navigator.plugins

Please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html

Thanks Jefferson, that worked like a charm. I usually try to find solutions "in browser", without having to install extensions, since my laptop is a pretty old one and I can't make firefox too heavy. But greasemonkey seems to be very light so this solution is perfect.

Sorry for the late reply and again many thanks for your work! I'll definitely use this script for other cases too!