Firefox is not made for windows XP try upgrading to windows 7! Here is How

You two different Flash plugins for Firefox on your system.

Shockwave Flash 13.0 r0
Shockwave Flash 23.0 r0 

Adobe Flash is notorious for not always removing old Flash Plugin DLL files as part of a Flash program update. Sometimes the user needs to uninstall Flash completely and get a clean start.

First, grab the Adobe uninstaller from here: http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html

2nd, get Flash here: https://get.adobe.com/flashplayer/ Make sure that you deselect the "Optional offers".

Run the Flash uninstaller program, then reboot your PC. Then install the Plugin version of Flash. So you'll have a clean installation of Flash for Firefox.

prestontoborg said

Firefox is not made for windows XP try upgrading to windows 7! Here is How

Wrong! Mozilla still supports WinXP, at least until next spring.

prestontoborg said

Firefox is not made for windows XP try upgrading to windows 7!

Firefox works on Windows XP SP2 (and SP3) all the way to Windows 10. However Firefox 53.0 and later will be dropping WinXP and Vista support. WinXP and Vista users can use the then Firefox 52 ESR for security updates for a while still as the 52.8.0 ESR will be out when Firefox 60.0 is released.

https://www.mozilla.org/firefox/50.0.2/system-requirements/

Note Microsoft does not provide the codecs needed to play HTML5 video for WInXP as they do for Vista and newer.

With some workarounds you may be able to. http://www.msfn.org/board/topic/175591-enable-mp4-h264-aac-html5-video-in-firefox-on-windows-xp-without-flash/