Yeah, see your Fedora XFCE memory usage is only 200+ MB. The Ubuntu memory usage in the shootout is over 900MB.
Crazy.
it is waste to not use RAM that is available
Hawing water in tap and not drinking it all is also waste?
depending on the region where you live you can't waste (cold) water, yes. I for one live in a region with many rain and luckily there is this process called "hydrologic cycle". If you need water, take it. This does not count for hot water, because you can waste energy easily ;-)
So, I live in a region when free RAM is now waste, because when I run IDE with FF, then I need more free RAM for them.
if the RAM is used by FF or an IDE it is not free ;-) I meant using _otherwise_ free RAM to speed things up is a good thing. When there is a high pressure on RAM you can easily free RAM that was only used to speed up things (caching, preloading, preallocated ram, ...). But free unused RAM doesn't help anybody.