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.

You can set the plugins.load_appdir_plugins pref to true on the about:config page to make Firefox scan the "<install>/plugins/" directory for plugins like the WMP plugin (np-mswmp.dll).

• http://kb.mozillazine.org/about:config

You can alternatively move plugins from "<install directory>\plugins\" to "<install directory>\browser\plugins\" to make Firefox find them again.

• http://www.ghacks.net/2013/05/15/why-you-may-have-lost-access-to-plugins-or-extensions-in-firefox-21/
• http://mike.kaply.com/2013/04/24/major-changes-coming-in-firefox-21/

Thank you for the reply. I should have added that I am an older person who has absolutely NO understanding of any but the most basic computer terms, and I have LOTS of trouble following written directions unless every step is spelled out in kindergarten language !!! Someone else (who's no longer around) installed this YouTube add on for me and I have no idea of how it was done...I just know, rote, the steps I follow to use it.

I couldn't follow Cor-el's reply because I do not understand what he/she was referring to. Such sentences as "plugins.load_appdir_plugins pref to true on the about:config page" are totally Greek to me. I have no idea where these things are in Firefox. I hope someone out there can tell me step by step in non-tech terms what to do !

Thanks

There is a support article which spells out the steps in lots more detail. The title gives one example of plugin that is affected, but the procedure is the same for any affected plugin. Hope this link helps: Windows Media or other plugins stopped working after Firefox update.