Use the key combination Ctrl+Shift+A (opens add-ons tab) and switch to plugins tab. There you can choose your preference for each plugin.

See also: *https://support.mozilla.org/kb/windows-media-or-other-plugins-stopped-working There has been a change in where Firefox searches for plugins and the plugins folder in the Firefox program folder is no longer scanned for plugins, so Firefox won't find them anymore if they are installed in the plugins folder instead of the "browser\plugins" folder that Firefox now scans. ---- *https://developer.mozilla.org/Add-ons/Plugins/Gecko_Plugin_API_Reference/Plug-in_Basics#How_Gecko_Finds_Plug-ins