There are a few things you can do to improve your performance on Older machines:

• Update Firefox: Update Firefox to the latest release. Newer versions often are significantly faster than older ones
• Update Windows. You should install every windows update, including service packs. It looks like you are on Windows 7 so it's not that old of an operating system
• Update your graphics Driver, Upgrade your graphics drivers to use hardware and WebGL
• Scan for malware that can slow down your computer: Troubleshoot Firefox issues caused by malware
• Try the steps at http://windows.microsoft.com/en-us/windows/optimize-windows-better-performance#optimize-windows-better-performance=windows-7

Let me know if this helps!

Thanks Tylerdowner! All things I have done. PC formatted and everything updated. Google chrome working fine for videos

Is the problem that videos don't play or just play slowly?

play too slowly

i can use mozilla for everything but not for videos!

PC formatted and everything updated! nothing helps. That since last mozilla update. old version was working fine for videos too.