uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Reinstall with Current Release Firefox 57.0 with a Full Version Installer https://www.mozilla.org/firefox/all/

Please let us know if this solved your issue or if need further assistance.

Did not work

Pkshadow said

uninstall Firefox. Then Delete the Mozilla Firefox Folders in C:\Program Files and C:\Program Files(x86) Then restart system. Then run Windows Disk Cleanup. Then run it again and click the button that says Cleanup System Files. Note: your Firefox Profile is saved. But you should make a back up before you do : https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles Reinstall with Current Release Firefox 57.0 with a Full Version Installer https://www.mozilla.org/firefox/all/ Please let us know if this solved your issue or if need further assistance.

Every video is audio only now. Here is the debug info.

{

 "ns": "yt",
 "el": "detailpage",
 "cpn": "weXyD4Gk3Fum--bF",
 "docid": "9mnTCYsbKfw",
 "ver": 2,
 "referrer": "https://www.youtube.com/watch?v=9mnTCYsbKfw&t=5s",
 "cmt": "6.573",
 "plid": "AAVeraJEyHrL-DWL",
 "ei": "KEYXWtXfBtK48wSGz4vwBA",
 "fmt": "247",
 "fs": "0",
 "rt": "16.382",
 "of": "rQXV6GcM2p0pBbAubXmoKw",
 "euri": "",
 "lact": 2,
 "cl": "176519171",
 "mos": 0,
 "state": "4",
 "vm": "CAEQARgE",
 "volume": 100,
 "subscribed": "1",
 "c": "WEB",
 "cver": "2.20171121",
 "cplayer": "UNIPLAYER",
 "cbr": "Firefox",
 "cbrver": "57.0",
 "cos": "Windows",
 "cosver": "10.0",
 "autoplay": "1",
 "hl": "en_US",
 "cr": "US",
 "len": "437.801",
 "fexp": "23700271,23701005,23701940,23702322,23702700,23704248,23705485,23705739,23706129,23707158,23708105,23708904,23708906,23708910,23708985,23709525,23710813,23711205,23711500,23711818,23712106,23712115,23712346,23713352,23713711,9422596,9431754,9449243,9452647,9466591,9471239,9474456,9474594,9476327,9477045,9480475,9480776,9483689,9484237,9484345,9485000,9487175,9488038,9489751",
 "afmt": "251",
 "vct": "6.573",
 "vd": "437.801",
 "vpl": "5.000-6.573,",
 "vbu": "0.000-42.709,",
 "vpa": "true",
 "vsk": "false",
 "ven": "false",
 "vpr": "1",
 "vrs": "4",
 "vns": "2",
 "vec": "null",
 "vvol": "1",
 "creationTime": 32182.49,
 "totalVideoFrames": 49,
 "droppedVideoFrames": 0,
 "corruptedVideoFrames": 0,
 "lct": "6.573",
 "lsk": false,
 "lmf": false,
 "lbw": "635507.405",
 "lhd": "0.125",
 "lst": "0.000",
 "laa": "itag=251,seg=4,range=573664-721848,time=40.0-50.0,off=0,len=148185,end=1",
 "lva": "itag=247,seg=7,range=6666014-7640276,time=37.4-42.7,off=0,len=974263,end=1",
 "lar": "itag=251,seg=4,range=573664-721848,time=40.0-50.0,off=0,len=148185,end=1",
 "lvr": "itag=247,seg=7,range=6666014-7640276,time=37.4-42.7,off=0,len=974263,end=1",
 "lvh": "r2---sn-ab5szn7d",
 "lab": "0.000-50.001,",
 "lvb": "0.000-42.709,",
 "ismb": 8870000,
 "relative_loudness": "-0.459",
 "optimal_format": "720p",
 "user_qual": "hd720",
 "debug_videoId": "9mnTCYsbKfw",
 "0sz": false,
 "op": "1",
 "yof": false,
 "dis": "none",
 "gpu": "ANGLE_(Intel(R)_HD_Graphics_Family_Direct3D11_vs_5_0_ps_5_0)",
 "cgr": true,
 "debug_playbackQuality": "hd720",
 "debug_date": "Thu Nov 23 2017 17:05:45 GMT-0500 (Eastern Standard Time)"

}

Since most Video is HTML5 now that is Video Card and See if you can find updates for your Intel(R) HD Graphics Family as they are old : driverDate: 8-4-2015 You can Google Intel(R) HD Graphics Family and find the download page https://www.intel.com/content/www/us/en/support/products/80939/graphics-drivers.html, but you may have to know what generation chipset you are using. CPU-Z would tell you this https://www.cpuid.com/ Can also try : https://www.intel.com/content/www/us/en/support/detect.html I do not see any errors in the drivers. Old & corrupted Drivers cause a wide range of issues. Please keep them updated to prevent issues and you never know when Intel will stop supporting them.

You may have crossed the threshold on age for your system vs tech. Try to get them updated.

Try ; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode

https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration TURN HARDWARE ACCELERATION OFF and see what happens.

Try making a new Profile and going to the same Videos that did not play. https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

Please let us know if this solved your issue or if need further assistance.