My system was recently upgraded to Mageia 5. The browser will not play any videos on youtube.com or military.com/video. The browser is Firefox ESR 38.1.0.
The error message is
This plugin is vulnerable and should be updated.
Activate Adobe Flash
Check for updates...
SOLUTION 
Make sure "plugincheck", shown below, says your flash player is up to date. Then click on "Activate Adobe Flash". A window pops up that has two buttons: "Allow Now", and "Allow and Remember". Click on "Allow and Remember". Shut down all browser windows, try again. (If that fails, log out, and reboot the system to guarantee a fresh start of the browser.)
plugincheckClicking on "Check for updates" takes the browser to
https://www.mozilla.org/en-US/plugincheck/ which shows the Adobe Flash Player as "Up to Date" with two different version identifiers: Shockwave Flash 11.2 r202, and 11.2.202.481.
DetailsFurther exploration takes me to a document at the Mozilla site
https://blocklist.addons.mozilla.org/en-US/firefox/blocked/p796 explaining the "Flash Player Plugin on Linux
11.2.202.424 and lower (click-to-play) has been blocked for your protection." and "All users are strongly recommended to update their Flash plugin." But my plugin is already at
11.2.202.481. It should be good.
On the same document is a link to "View block request". Clicking on it will take the browser to
https://bugzilla.mozilla.org/show_bug.cgi?id=1109795 A part of that document says,
We have a problem on Linux, since we use regular expressions in the plugin description to check for version numbers, and the description strings look like this: "Shockwave Flash 11.2 r202". So, we don't have the 4th part of the version string. We use the description because of some problem that caused the version number field to be unreliable on Linux, but I don't remember the details, or know if it's still a problem. In the current state of things we could block up to either 11.2.201.* or 11.2.202.* on Linux.