More than likely a graphics driver issue. See if this helps you: https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

And if that doesn't work or your drivers are the latest available, try disabling Hardware Acceleration and / or WebGL.

Would have been nice to have your Troubleshooting Information as requested when you started this thread. That info might have given us a hint to the cause ...

VIDEOS PLAYING SOUND ONLY AND A BLACK SCREEN IN CHROME...For those using Chrome and not finding all these settings under options, accounts, security, etc., that everyone is suggesting, I was having the same problem. I found a solution. I went to play the video by clicking it, when it opened, I had the black screen and sound. RIGHT CLICK>SETTINGS> UN-CHECK "ENABLE HARDWARE ACCELERATION" I kept trying to click the box to un-check it, but nothing was happening and I couldn't click the other tabs on the menu or close it. I finally found that I could click just to the left of the box, on the edge of it almost, and it UNCHECKED THE BOX. Then it let me close the box and I restarted the video, IT WORKED!

the-edmeister said

More than likely a graphics driver issue. See if this helps you: https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration And if that doesn't work or your drivers are the latest available, try disabling Hardware Acceleration and / or WebGL. Would have been nice to have your Troubleshooting Information as requested when you started this thread. That info might have given us a hint to the cause ...

Updated to 355.82 and tried disabling WebGL (hardware acceleration is off by default) but still only get the black screen on twitch.tv.

Troubleshooting data:

{

 "application": {
   "name": "Firefox",
   "version": "42.0a2",
   "buildID": "20150901004009",
   "userAgent": "Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:42.0) Gecko/20100101 Firefox/42.0",
   "safeMode": false,
   "updateChannel": "aurora",
   "supportURL": "https://support.mozilla.org/1/firefox/42.0a2/WINNT/en-US/",
   "numTotalWindows": 1,
   "numRemoteWindows": 1,
   "remoteAutoStart": true
 },
 "lockedPreferences": {},
 "graphics": {
   "numTotalWindows": 1,
   "numAcceleratedWindows": 0,
   "windowLayerManagerType": "Basic",
   "windowLayerManagerRemote": true,
   "supportsHardwareH264": false,
   "numAcceleratedWindowsMessage": [
     ""
   ],
   "adapterDescription": "NVIDIA GeForce GTX 980",
   "adapterVendorID": "0x10de",
   "adapterDeviceID": "0x17c8",
   "adapterSubsysID": "49983842",
   "adapterRAM": "4095",
   "adapterDrivers": "nvd3dumx,nvwgf2umx,nvwgf2umx nvd3dum,nvwgf2um,nvwgf2um",
   "driverVersion": "10.18.13.5582",
   "driverDate": "8-25-2015",
   "adapterDescription2": "",
   "adapterVendorID2": "",
   "adapterDeviceID2": "",
   "adapterSubsysID2": "",
   "adapterRAM2": "",
   "adapterDrivers2": "",
   "driverVersion2": "",
   "driverDate2": "",
   "isGPU2Active": false,
   "direct2DEnabled": false,
   "directWriteEnabled": false,
   "directWriteVersion": "6.1.7601.17514",
   "direct2DEnabledMessage": [
     ""
   ],
   "webglRenderer": "Google Inc. -- ANGLE (NVIDIA GeForce GTX 980 Direct3D9Ex vs_3_0 ps_3_0)",
   "info": {
     "AzureCanvasBackend": "skia",
     "AzureSkiaAccelerated": 0,
     "AzureFallbackCanvasBackend": "cairo",
     "AzureContentBackend": "cairo"
   }
 },
 "javaScript": {
   "incrementalGCEnabled": true
 },
 "accessibility": {
   "isActive": false,
   "forceDisabled": 0
 },
 "libraryVersions": {
   "NSPR": {
     "minVersion": "4.10.9",
     "version": "4.10.9"
   },
   "NSS": {
     "minVersion": "3.19.3 Basic ECC",
     "version": "3.19.3 Basic ECC"
   },
   "NSSUTIL": {
     "minVersion": "3.19.3",
     "version": "3.19.3"
   },
   "NSSSSL": {
     "minVersion": "3.19.3 Basic ECC",
     "version": "3.19.3 Basic ECC"
   },
   "NSSSMIME": {
     "minVersion": "3.19.3 Basic ECC",
     "version": "3.19.3 Basic ECC"
   }
 },
 "userJS": {
   "exists": false
 },
 "extensions": [
   {
     "name": "Adblock Plus",
     "version": "2.6.10",
     "isActive": true,
     "id": "{d10d0bf8-f5b5-c8b4-a8b2-2b9879e08c5d}"
   },
   {
     "name": "Classic Theme Restorer",
     "version": "1.3.8",
     "isActive": true,
     "id": "ClassicThemeRestorer@ArisT2Noia4dev"
   }
 ],
 "experiments": [],
 "modifiedPreferences": {}

}