I'm assuming this problem seems to be caused most preferably by hardware acceleration. Or probably internet speed issues.

No, other browsers Ihave don'thave this issue.

Can you please provide the specifications of your mobile phone so that we can help you better. Thank you.

Galaxy 4 Tablet Model No. SM-T530NU Android Ver. 5.0.2 Kernel Ver. 3.0.4-5816452 dpi@SWHD4004 #1 Build No. LRX22G. T530NUUEU1BOI2 KNOX Ver. KNOX 2.4

                  Standard SDK 5.4.0
                  Premium SDK 2.4.0
                  Customization SDK 2.4.0
                  Container 2.4.0
                  CEP 2.0.1
                  OTP 1.1.0
                  SE for Android 2.3.0
                  SSO 2.4.0
                  TIMA 3.0
                  VPN 2.2.0

Thank you for providing the information. As i see, YouTube doesn't allow higher resolutions (or lower resolutions) to many browsers and i assume Firefox is one of them. You can use Google Chrome and it works perfectly fine on them because well they both are products of Google.

I suggest you to the use the mobile version of YouTube : m.youtube.com for the best experience with Firefox.

Please rate "yes" if this helped you :)

I always had on the mobile version. Thanks anyway.

We will take this bug into consideration and this issue will be taken up to the concerned authorities. Till then, unfortunately you have to use Chrome or other supported browsers. Thankyou.