Firefox can play YouTube, but can't open mp4 from pc?

Videos play perfectly fine when i visit YouTube.com; I mean, i got the latest Flash plug-in, so i expect the browser to play Flash-related videos. But, when i open mp4 or flv files by clicking File -> Open FIle (Ctrl + O), these files are not playing at all!!!! If i right-click on the screen, it shows menu about QuickTime plug-in. This is driving me crazy!!!! I have a presentation at school, and that computer won't let me change the settings. How is this even possible?

When you load a video on YouTube, Firefox starts the Flash plugin with YouTube's SWF player. The SWF player imports and plays the actual MP4 or FLV media. Firefox doesn't associate Flash with these formats directly.
Can you use a stand-alone player, such as QuickTime or VLC to play the downloaded file outside of the browser?
If you need to play it in the browser, perhaps someone has posted instructions on how to do it. Because of security restrictions, you probably would need to save a web page file (.html) and a SWF player into the same folder as the media in order to get everything to work together.

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