Windows – Unable to play mp4 files embedded in a webpage

There's this MP4 file that I can play in Firefox 47 on Windows Vista SP2 if I access it directly:

http://www.quirksmode.org/html5/videos/big_buck_bunny.mp4

The website I'm trying to access, however, serves up MP4 files embedded in HTML with the video tag and the source tag. e.g..

<video autoplay="true" controls="true" height="520" width="900">
    <source type="video/mp4" src="http://www.quirksmode.org/html5/videos/big_buck_bunny.mp4">
</video>

Here's a jsfiddle of it:

https://jsfiddle.net/ke55q564/2/

So when the MP4 is loaded that way it doesn't play at all. Instead I get a "No video with supported format and MIME type found." error where I'd expect the video. The JS console elaborates:

Specified "type" attribute of "video/mp4" is not supported. Load of media resource http://www.quirksmode.org/html5/videos/big_buck_bunny.mp4 failed.

Anyway, I don't get it and I don't know what to do about it. This works in Firefox and Google Chrome on my Windows 10 laptop but it's not working on this newly reformatted Windows Vista laptop.

Normally I'd install Google Chrome but apparently Google Chrome now requires Windows 7. And downloading older versions of Google Chrome that support Windows Vista doesn't seem to be an option.

So I don't know what to do. Do I have to spend a few hundred dollars on Windows 10 just to hope that it'll work on this Windows Vista-era laptop? Should I just forget about trying to do it with Windows at all and just install Ubuntu? Should I just write the laptop as a piece of junk and buy a brand new one?

I have the latest version of VLC installed and the FF plug-in from VLC installed and activated as well..

I don't know what to do. Any ideas?