I haven't heard of any similar message before. In your "More system details..." it lists two different Adobe PDF plugins, version 8 and version 9. You may have a conflict there, especially if the older one isn't disabled. You could try to disable it here:

orange Firefox button or Tools menu > Add-ons > Plugins category

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If that doesn't help... Adobe Reader and the plugin have been updated to version 10. but Are you willing to upgrade Adobe Reader to see whether that fixes the problem?

http://get.adobe.com/reader/

Exact same problem here.

Firefox claims Adobe is already running and IT IS NOT!

I am using VISTA (CRAP) sp2.

I have had nothing but problems with VISTA. It will NOT network with XP or anything else XP. It is the most horrible OS from Micro$oft ever! Win 3.1 was OK, Win 98 was better, & Win XP was good except for not being able to do multi-session CD's.

So we either have a Firefox problem or it is the OS.

Now you've heard of a second identical issue.

My solution was to right-click and download the PDF. Then open it in Adobe Reader.

There are other issues with this new Firefox that I did not have with the older version but they are tolerable.

Problem fixed. I disabled old versions of adobe reader plug in and downloaded Adobe Reader X. Works fine now.

Cannot find Adobe Acrobat/Reader in the add-ons.

I have v9 & horrible M$ VISTA and it does NOT work with Firefox 5.

Had same problem. Followed all Firefox recommendations about plug ins etc etc with no result. In the end uninstalled Acrobat Reader (think it was v9) and re-installed from their website ( Adobe Reader X (10.1.0) ). All good now. Took seconds, wish I'd done that first.