Adobe might not have released one, or may not be including it with the standard install. At least, I do not see it advertised in the registry on my system:

HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\MozillaPlugins => has an entry for Adobe Acrobat for the 32-bit plugin

HKEY_LOCAL_MACHINE\SOFTWARE\MozillaPlugins => does not have an entry for Adobe Acrobat

HugoLudwig said

The Adobe Reader Plugin is simply not available in my Firefox 38.05 64 bit- I can't download it from Adobe (tried it) or anywhere else - how can I get the Plugin??? Thank you

HugoLudwig said

The Adobe Reader Plugin is simply not available in my Firefox 38.05 64 bit- I can't download it from Adobe (tried it) or anywhere else - how can I get the Plugin??? Thank you

That is the downside with trying to use any Win64 Firefox builds (including the 38.0.5b2 you are testing) is the plugins needs to be 64-bit as 32-bit Plugins will not work.

Fortunately, Firefox has a built-in PDF viewer, and the download button on the toolbar of the viewer makes it fairly easy to launch PDFs in your external Adobe software if the viewer doesn't handle the PDF well or lacks features important to you.

James said

That is the downside with trying to use any Win64 Firefox builds (including the 38.0.5b2 you are testing) is the plugins needs to be 64-bit as 32-bit Plugins will not work.

Thank you for the clarification - if no plugin is available for 64 bit this is fine with me - but I will stay with FF 64 bit . Again thanks for the clarification.