Hello, this is my first post in this forum.
I just fresh installed Manjaro KDE last night but I got a problem with Youtube fullscreen on firefox, it appears extra black space on top and this problem just exclusively happens on youtube, I’ve tried fullscreen on other sites like twitch but it works fine.
Anyone could help me with this problem?
Welcome to Manjaro!
Please read this:
https://archived.forum.manjaro.org/t/how-to-provide-good-information-in-your-posts/37102
and post some more information so we can see what’s really going on. Now we know the symptom of the disease, but we need some more probing to know where the origin lies…
An inxi --admin --verbosity=7 --filter --no-host would be the minimum required information… (Personally Identifiable Information like serial numbers and MAC addresses will be filtered out by the above command)
and also read that:
as you violated the most important rule of posting screenshots: never post text as a screenshot! 
Also are you using cinema mode (diagonal arrow below) or full screen mode (vertical arrow below)?
In cinema mode, you will get these black bars whereas in full screen mode, it depends on the movie itself: all videos having the same aspect ratio as your monitor (16:9 for HD resolution in your specific case) will not.
