How to make Firefox buffer/preload full videos (HTML5)

Open about:config. Set media.mediasource.enabled to false.

Some sites use Mediasource, a web standard that allows to load videos in chunks. It can be disabled as selayang said, but that’s not all.
Firefox uses the following prefs to buffer only part of a video:
media.cache_readahead_limit, which determines how many seconds of the video need to be downloaded
media.cache_resume_threshold, which specifies at how many seconds from the end of the downloaded part of the video buffering should resume. If a video is 20 minutes long and Firefox downloaded 12 minutes of it, this pref being set to 120 will mean buffering will resume when you reach the 10th minute.
If you want to buffer everything, set media.cache_readahead_limit to something like 99999.
I don’t think this should apply to websites that use Mediasource, so you would need to disable it and hope they don’t have a fallback. Sites with Mediasource are not that numerous, usually it’s the big ones like YouTube, Dailymotion, Twitter, … Sites with a fallback that would prevent media.cache_readahead_limit from controlling buffering should be even more rare I believe. The new YouTube might have that.

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