Firefox is not shipped by default in Kubuntu (they do shipped in Ubuntu), so in order to use it, i have to install it using apt-get, but that's when the problem arose. Since i'm using Indonesian time zone, the installer create a sources.list file that contains a link to Indonesian site, such as http://id.archive.ubuntu.com. The problem with is that sometimes, Indonesian sites are inaccessible, so i couldn't get anything when i ran apt-get update. The solution is to change the id into us and re-ran the command and voila..... everything works like charm. I can install Firefox again
The default resolution that was picked during installation was so high. It was about 1600x1200 if i'm not mistaken. I choose to reduce it to normal resolution, 1024x768 using KDE Control Center. If i don't do this, i won't be able to have full screen mode.
As usual, Ubuntu never shipped codecs, so people will have to download some package from the repositories when they want to listen to musics or songs or videos. Lucky me, Slackware by default shipped this application, so i can start listening to mp3 files out-of-the-box.