Did you check at the very bottom of the Plugins panel of the Add-ons page? Disabled plugins usually sort to the end of the list, out of alphabetical order.

If you've never installed Flash on your Windows 10 system, then you may need a fresh installer direct from Adobe. On the following page, there often are optional offers for other companies' software. Make sure the uncheck any unwanted extras:

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

If Shockwave Flash STILL doesn't show up on the Plugins panel of the Add-ons page, then we can work through a number of possible reasons that Firefox isn't finding it.

Note Flash Player is listed as Shockwave Flash in the Plugins pannel of Addons Manager. So it is the same Plugin.