Windows XP - Firefox - Enable MP4

about:config
  media.gmp-eme-adobe.enabled        boolean true
  media.gmp-eme-adobe.forceSupported boolean true
  media.gmp-eme-adobe.visible        boolean true
  media.gmp-eme-adobe.version        string  17
  media.gmp-eme-adobe.abi            string  x86-msvc-x86
  media.gmp-eme-adobe.lastUpdate     integer 1500000000
  media.gmp.decoder.enabled          boolean true
  media.eme.enabled                  boolean true
  media.gmp-provider.enabled         boolean true

about:support > Profile Folder > Open Folder
Create Folders gmp-eme-adobe/17
Extract primetime_gmp_win_x86_gmc_40673.zip

about:config
  media.cache_readahead_limit        integer 0

Edit