Okay, so during my searches I didn't found anything due to my restricted accesses. I'm gonna continue this evening. But in the case where I really find nothing, I would advice you to contact the addon's editor on its website which is sling.com if i'm not wrong. IMO, I think it would be the best solution.

Try this:

1. Right-click this hyperlink - http://plugin.slingbox.com/downloads/pc/1.5.1.119/webslingplayer.xpi - and use Save Link As... to save it to your drive.
2. Then drag that XPI file into the Firefox browser window to install it. Or, you can use (File) > Open File and navigate to where you saved that file, then use Open to install it.

edited for formatting

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.

You can alternatively move plugins from "<install directory>\plugins\" to "<install directory>\browser\plugins\" (create this folder when missing) 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/

See also:

• https://support.mozilla.org/kb/windows-media-or-other-plugins-stopped-working