Look if you use any add-ons in firefox that could prevent the playback. Update all of them if possible or needed. Try again. If thats not working, try disable them one by one to determin if thats the problem. If that does not help a restart and I mean full restart could help, not sleepmode or the Windows restart function. When that does not do the trick, look if you have changed the default Internet settings and try to reset them. Last but not least if you use a VPN that could intervere too. Without more knowlege about your problem in detail thats all I can give you as an advice.