Installing ceases at 47%

I have a Mac on Mountain Lion, primarily using Safari (yes I be a noob) and it has run Adobe Flash comfortably for many years. Only today I found a notice to re-install, to find that I could not complete it, with the installation dawdling to 7% followed by a notice that the process has ceased at 47%. I tried a few different methods, apparently my firewall is too restrictive, but none of them seem catered to a Mac, or if so were too complicated for me to understand. Any help would be much appreciated, I can't view any videos at all ahaha thanks in advance