The problem is quite new. I can't tell exactly when it appeared first time. It could be Firefox version 61.0 only, or also earlier version before that. It worked perfectly earlier, and there are no changes on my hardware or software, excluding Linux updates and Mozilla updates. Problem shouldn't either be on the Internet connection. I have 100 mbps connection, and the problem exists also in low quality video (240p etc.) . There is no buffering either. All video is lagging a lot. Picture could seize for couple of seconds, or playing like 5 - 10 fps. Audio plays normal. This occurs in Youtube and Netflix, and probably in other platforms also. On Windows (same computer) there is no problem (Chrome, older Firefox). Details: Linux Mint 18.2, Firefox 61.0 64 bit, Plugins: Open H264 video codec, Widevine content decryption module (drm-module) Any ideas what might cause the problem?