As you can see (attachment 1), the "Memory cache device" was showing only 5,5 MB.

So how could about 1 hour of 720-pixel HD video material take up only 5,5 MB? This means (and hard disk activity confirmed this) that it was written to disk.

What's more the figure shown in Options > Advanced > Network (attachment 2) should be the same as the one shown in about:cache. It is not but in previous version (Firefox 30.0) it was.

Can anyone tell me how that exactly works?

attachment 2:

The memory cache is not used for all kinds of data, so if you disable the disk cache then Firefox won't store necessarily store data in the memory cache that was previously in the disk cache.

Videos from YouTube are likely loaded in chunks by the Flash player and not as a full video, so you may only see the last or current chunk in the memory cache (those are likely not saved to the disk cache anyway even when enabled).

"if you disable the disk cache then Firefox won't store necessarily store data in the memory cache that was previously in the disk cache"

So it will store it on the disk despite the setting, right?

"Videos from YouTube are likely loaded in chunks by the Flash player"

I was just saying, I am not using Flash (not installed). I watched the videos through HTML5. But will install Flash to see what you mean.

So in my case then (disk cache disabled), the phrase "Your web content cache is currently using 0 (it always shows zero) bytes of disk space" in Options > Advanced > Network refers literally to the disk itself, hence not to RAM or RAM+Disk?

All I am trying to achieve is a minimum hard disk wear on a computer where streaming videos/audio is used A LOT.