Please 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. (Note: This should be Pinned and run Weekly, If never done below expect 10's of gig's) 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
• https://support.mozilla.org/en-US/kb/export-firefox-bookmarks-to-backup-or-transfer

Reinstall with Current Release Firefox 57.0.4 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.

DENISEVE said

When I tried to play a video on CNN, I got an error message stating that the VIDEO FORMAT OR MIME TYPE IS NOT SUPPORTED. I tried downloading different versions of media feature packs but they all say that they're not compatible with my computer. I'm running Windows 10, Firefox Quantum 57. Anyone have the same issue or know how to correct it? thanks

The media feature pack are for if you have a N or KN version of Windows Vista, 7, 8, 10 https://support.mozilla.org/en-US/kb/fix-video-audio-problems-firefox-windows

You can see if you fully support HTML5 player at say https://www.youtube.com/html5/ as all check marks should be blue.

You can also test video examples at https://www.quirksmode.org/html5/tests/video.html (ignore the old table on top).