It’s very easy to fix this issue / error that you get in Firefox. It takes 5 minutes.
Follow these steps
Click on the Firefox tab (left top for Windows 7+), click on addons.
Then click on “plugins”.
Scroll down for “shockwave flash”. Click on “update now”. Then on the next window, click on “plugin check page” > update now > install now.
Download and install Adobe Flash.
That is all. After this, you can check “Firefox”, addons and see that the red marking for Adobe Flash is no more.