A reboot of the machine also doesn't resolve the issue.

I have closed Safari (Chrome, Firefox) checked again and still no extension. If l look in Safari -> Preferences -> Extensions there is no sign of the SfB Plugin. I have tried Firefox, Chrome and of course Safari and after a while, I get the dreaded Yellow bar stating: 'Skype for Business Web App Plugi-in installation has timed out or failed.'

We heavily rely on SfB meetings at my organisation and I am having countless complaints about Mac users unable to join audio or video from their Mac's as the SkypeforBusinessPlugin.pkg file served by our on premise pool server doesn't appear to do much apart from 'successfully' installing according to the installer.