Web developer.
Even if you’re still running macOS High Sierrra (I’m on 10.13.6) you’ll receive a Safari update (from 12 to 13). This will break a number of extensions – including most adblocking – also a vim-based keyboard extension I was using called Vimmy.
So you may want to avoid upgrading, or do as I’ve done and switch your default browser to Firefox etc. for the time being.
I’ve routinely found the standard Adobe Flash player updater for Mac (OS X/macOS) fails right at the end, without explanation.
However there is a version that always works. Go to:
https://helpx.adobe.com/flash-player/kb/installation-problems-flash-player-mac.html
… and choose Flash player for Safari and Firefox – NPAPI – the download URL is normally:
https://fpdownload.macromedia.com/pub/flashplayer/latest/help/install_flash_player_osx.dmg
The opening dialog box should look a bit different – like this:
Use this version (last tested 10 Feb 2017):
https://fpdownload.macromedia.com/pub/flashplayer/latest/help/install_flash_player_osx_ppapi.dmg
This is a very fast and handy way of seeing if your copy of Flash or Silverlight is up to date.