Please try the offline installer posted at the bottom of the Installation problems | Flash Player | Windows page, in the 'Still having problems' section. There will be three (3) installers. Since you're on Windows 8.x, ignore the installer for ActiveX Control, as Microsoft embeds Flash Player in Internet Explorer on Windows 8.x systems. You'll want to download the 'Flash Player for Firefox - NPAPI' installer file. If you also use Opera browser you'll want to download the 'Flash Player for Opera - PPAPI' installer file.