The site is using Youtube videos. Youtube is using a HTML5 player like most sites now days for video, audio and games even. The Flash Player Plugin has not been supported by both Adobe and web browsers for over two years now so that is not an option for WinXP. The issue is you are using the old EOL WinXP. Microsoft does not support Windows XP, only Vista (well did support) and later with the media feature packs (that provided the codecs needed) so you are going to only partially support HTML5 players. You may say the ancient Chrome version you tried seem to work, well Chrome kind of cheats by coming with its own codecs. If you want to both have much more current Firefox and full support of HTML5 players then perhaps dual booting with a light 32-bit Linux distro may be an option. However still supported 32-bit Linux distros are becoming more scarce as less people still use the ancient 32-bit CPU's over time.