You may have seen this headline recently:
Firefox blocks Flash, and Facebook calls for its death

Firefox has been blocking outdated versions of Flash for quite some time now. This isn’t news; it’s click-bait.

You can update the version of Flash that you do have by visiting https://get.adobe.com/flashplayer/

You also can test/check to see what version you currently have installed at https://www.adobe.com/software/flash/about/

Update to the newest version of Flash, version 18.0.0.209 (as of this moment), and Firefox will remove the block automatically.