(Edit 1: Additional Information) (Edit 2: This problem is now resolved. I don't know how, but after I installed Lubuntu and Xubuntu alongside my Ubuntu, the problem was gone before I realized.) Yesterday, I got a new Dell Inspiron and installed Ubuntu 20.04 there, and as usual it comes with Firefox pre-installed. But when I tried to use Firefox to play videos (for example, from nicovideo.jp), it gives me an error message. I tried some streaming sites, but apparently only Youtube that worked without problem. Another problem that I encountered is Google Play Music website requiring me to install Adobe Flash Player to play musics. Some things that I've tried so far but failed to solve the problem: # Disabling Firefox hardware acceleration. # Uninstalling all add-ons. # Uninstalled and re-installed Firefox. # Started Firefox in safe mode (running firefox -safe-mode in terminal). Additional informations: # Usually I use Firefox on my ASUS TUF (my OS is also Ubuntu 20.04) for almost all of my internet needs including playing videos and listening to musics. It works just fine with the same setting (I sync my Firefox across all my devices). I remember didn't do anything special to set the Firefox on my ASUS; I just use the pre-installed one and the videos and musics are playing just fine. The software I installed are also pretty much the same. # On my Dell, I tried to play videos and Google Play music using Google Chrome, and they work just fine. The videos are playing and Google Play Music didn't require Adobe Flash Player. # Solutions provided here ([[https://support.mozilla.org/en-US/questions/1208656]]) didn't work. # I installed Lubuntu and Xubuntu 20.04 on top of my Ubuntu, and all video streaming site and Google Play Music worked on both distro's Firefox. So this problem only persists on my Ubuntu's Firefox. Feel free to ask for additional information. I'll gladly provide it if needed.