That is an ad. Don't worry about it.

But you do have a problem.

Separate Issue; Your System Details shows;

Installed Plug-ins

Shockwave Flash 19.0 r0 Shockwave Flash 29.0 r0
Having more than one version of a program may cause issues. You must remove the older programs. Then get the current full installer.

Flash: Grab the uninstaller from here:

http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html Uninstall Flash Player | Windows

http://helpx.adobe.com/flash-player/kb/uninstall-flash-player-mac-os.html Uninstall Flash Player | Mac

Then reinstall the latest version.

Note: Windows users should download the active_x for Internet Explorer. and the plugin for Plugin-based browsers (like Firefox).

Note: Windows 8 and Windows 10 have built-in flash players and Adobe will cause a conflict. Install the plugin only. Not the active_x.

Flash Player Version 29.0.0.171

https://get.adobe.com/flashplayer/ Direct link scans current system and browser Note: Other software is offered in the download. <Windows Only>

https://get.adobe.com/flashplayer/otherversions/ Step 1: Select Operating System Step 2: Select A Version (Firefox, Win IE . . . .) Note: Other software is offered in the download. <Windows Only>

See if there are updates for your graphics drivers https://support.mozilla.org/en-US/kb/upgrade-graphics-drivers-use-hardware-acceleration

Whistlermade said

I'm using ESR 52.8.0 (64-bit) and get a tab on http://www.fnvfox.com/thankyou.html saying "Download Flash & Video". I've updated plug-ins yet!!

The page is due to the Download Flash and Video extension you installed, not a plugin.

https://addons.mozilla.org/firefox/addon/download-flash-and-video/

Extensions for Firefox 57+ have reduced ability to operate outside of specific folders on your system. Some extensions now require a completely separate program from their publisher to work around this. The extension coordinates with the external program (native application) to gain more privileges on the system. Mozilla does not review these separate programs.

Since you are running Firefox 52 ESR, you may find it more convenient to use an older "legacy" version of the extension that contains all the functionality within the extension. Can you figure out which one updated to the new approach? Something related to video downloading...