Safari uses a different version of Flash than Firefox would use. Download Flash from Adobe while using Firefox, then install it after closing Firefox.

https://get.adobe.com/flashplayer/

I have tried this solution many times - Mixbook still gives me same prompt to download Adobe Flash

https://support.mozilla.org/en-US/kb/keep-flash-up-to-date-and-troubleshoot-problems#w_installing-or-updating-flash

If Firefox was still running when you installed the Flash Player then either restart Firefox or you could try typing about:plugins in Location (address) bar so Firefox will scan for Plugins on system.