Mozilla is soft blocking (click to play) the previous versions of Flash player due to proven in wild critical exploits.

Mozilla is not soft blocking any of the current versions which are considered safe for now.

http://www.adobe.com/software/flash/about/ https://www.adobe.com/products/flashplayer/distribution3.html https://addons.mozilla.org/en-US/firefox/blocked/

Since you have Windows you may want to unistall Flash first. Why well some users have discovered the previous version was still there after installing the current version and this can cause Firefox to still make use of previous version. https://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html

Flash 16 is obsolete now...

If you are not accustomed to using the "Ask to Activate" feature with any of your plugins, here's how it works:

When you visit a site that wants to use Flash, you should see a notification icon in the address bar and usually (but not always) one of the following: a link in a black rectangle in the page or an infobar sliding down between the toolbar area and the page.

The plugin notification icon in the address bar typically looks like a small, dark gray Lego block. When the page wants to use a soft-blocked plugin, the icon turns red to alert you to the concern.

If you see a good reason to use Flash, and the site looks trustworthy, you can go ahead and click the notification icon in the address bar to allow Flash. You can trust the site for the time being or permanently.

But some pages use Flash only for tracking or playing ads, so if you don't see an immediate need for Flash, feel free to ignore the notification! It will just sit there in case you want to use it later.