See if this helps you:
http://www.ghacks.net/2013/05/15/why-you-may-have-lost-access-to-plugins-or-extensions-in-firefox-21/

Thanks, information is good but I need a step by step if possible.

Maybe this article is clearer? Windows Media or other plugins stopped working after Firefox update

Another possibility is Firefox's new ability to natively play MPEG media using Windows Media components. This may interfere or conflict with other players. To test that possibility, you can disable this feature as described in this thread: How to disable the built-in media player in FFv21?.

Changing the media field to false doesn't work either.

Can you provide a link to a page that uses the DivX player that isn't working?

link text

If you use extensions (Tools > Add-ons > Extensions) like Adblock Plus or NoScript or Flash Block that can block content then make sure that such extensions aren't blocking content.

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes