Hi, You can try deleting the pluginreg.dat file in the [http://kb.mozillazine.org/Profile_folder Firefox Profile Folder] after exiting Firefox. You can also open the profile folder via Firefox '''Help''' ('''Alt''' + '''H''') > '''Troubleshooting Information''' > '''Show Folder'''. You can also try to '''Disable''' all the versions except the one working version in '''Tools''' ('''Alt''' + '''T''') > '''Add-ons''' > '''Plugins'''. [https://support.mozilla.org/en-US/kb/use-plugins-play-audio-video-games?redirectlocale=en-US&redirectslug=Using+plugins+with+Firefox Using plugins] [http://kb.mozillazine.org/Profile_folder_-_Firefox Firefox Profile Folder & Files]

I tried both things but still have no luck. Thanks anyway.

Please also see [https://support.mozilla.org/en-US/kb/quicktime-plugin-play-audio-and-video?redirectlocale=en-US&redirectslug=Using+the+QuickTime+plugin+with+Firefox this]. [http://kb.mozillazine.org/Video_or_audio_does_not_play Video or audio does not play] [http://kb.mozillazine.org/Issues_related_to_plugins Plugin issues] [http://kb.mozillazine.org/Testing_plugins Testing plugins]

All those QuickTime plugins handle different MIME types, so you need all those plugins. You can verify that on the about:plugins page. *http://kb.mozillazine.org/Issues_related_to_plugins