First, sory for my English is a shame but I don't know it well. For a half year (I think it was so long) I used Firefox on the control of Fedora 23 operating system. I have no problem with play video in html5 (only some problem with image - on some sites appeared a disruption - a horizontal line which move from up to down) . I was forced, however, to re-install my two operating systems because of windows problems. When I did it when I install Fedora afresh I realize that Firefox doesn't play videos on onet.pl, interia.pl, cda.pl, filmweb.pl. From the whole sites I used to visit only youtube work well. So, because of this problem I tried to identify the problem, so I installed Ubuntu and I sow that in this system everything work well. On Windows videos work well too. So I check the another computer in my home (my dad's) where is installed Fedora 23 and I have seen that everything work well. I don't understand why on my computer, in the past, videos worked and now they don't on the control of the same system. Now I use Fedora 25 but I checked if the videos work on Fedora 23 after newly installed (without updates) because now Fedora 23 have only one big update and nothing more. But the question was: if the videos worked on the same system installed from the same media in the past why they doesn't work now on the same computer? I find the site youtube.com/html5 where is tell what kind of element needed to watch videos in html5 work properly on my computer. It tell that on my browser (Firefox) the elements H.264 and MSE & H.264 doesn't work The curious thing is that Firefox on my dad's computer doesn't have an active OpenH264 but the site youtube.com/html5 tell that every of elements run properly. Firefox on my computer (with the problem) have this codec activated but the site show what I tell - two elements are not work properly. On Chrome on Fedora the videos work well and every element is marked in blue color.