A long shot, but if that If that plugin doesn't handle "video/divx" then the latest DivX Web Player plugin may have been copied to the Firefox\plugins folder (C:\Program Files (x86)\Mozilla Firefox\plugins by default, on Windows 7 64-bit). Firefox no longer scans that location (ref.). To enable any plugins in the Mozilla Firefox\plugins folder, you could open about:config from the Firefox address bar, double-click the plugins.load_appdir_plugins preference to change it's value to true and then restart Firefox. See Windows Media or other plugins stopped working after Firefox update for details.