It could be that certain files are still open and in use, which could prevent the installation
Please provide any error messages?
Usually the easiest for some users is to download all the files then re-boot , but that is not necessary
You can open an elevated cmd prompt ( How to Open the Command Prompt as Administrator in Windows 8.1 )
then right click copy/paste in the black window one line at a time IF an error shows, ignore it meaning that the process is not open:
taskkill.exe /im plugin-container.exe /f
taskkill.exe /im firefox.exe /f
taskkill.exe /im flash* /f
Some of that is not well outlined in these forums but having some or all of those open could cause the install to fail, I have seen this numerous times where users say everything is closed but there are lingering processes open that are part of Firefox and flash.
Then try the Flash installer for Firefox here: ( https://fpdownload.adobe.com/pub/flashplayer/pdc/18.104.22.168/install_flash_player.exe )