When does that box pop up -- is it when you start Firefox or visit a particular site or ... ??

To double-check whether Firefox is set to start up in a backwards compatible mode, you can access the Properties dialog for the shortcut. Either:

• right-click a desktop shortcut
• right-click a pinned taskbar icon, then right-click Mozilla Firefox

In the Properties dialog, click the Compatibility tab and make sure nothing is set/selected on this tab. (Screen shot attached.)

There are other things that need attention.

The System Details List shows that you have multiple Flash plugins.

1. Shockwave Flash 15.0 r0
2. Shockwave Flash 20.0 r0

You can find the installation path of all plugins on the about:plugins page.

• http://kb.mozillazine.org/Issues_related_to_plugins#Identifying_installed_plugins

You can check the Flash player installation folder for multiple Flash player plugins and remove older version(s) of the plugin (NPSWF32) and possibly (re)install the latest Flash player.

• (32 bit Windows) C:\Windows\System32\Macromed\Flash\
• (64 bit Windows) C:\Windows\SysWOW64\Macromed\Flash\