Hi

What is the video that you are trying to play? This will help me to look into this here and hopefully find a solution for you.

any video from firefox on android with s9 plus sm-g965f, this happen and in latest firefox because i have update today

Do you have a strong WiFi connection when you try to watch a video?

maybe is not wifi connection problem for this problem because i can open any video from youtube on samsung browser or chrome browser, any way thanks for your response for the problem.

Do you have any add-ons installed in Firefox for Android?

i have adblocker adguard and in samsung browser but not have this problems

i resolve this problem after a storage clean data in app info and reopen again installing adguard and everything is ok now