NT 4.10 in the user agent is clearly showing that you are running Firefox in compatibility mode for Windows 98, so you need to check this like I posted above.

• User Agent: Mozilla/5.0 (Windows NT 4.10; rv:37.0) Gecko/20100101

This error usually means that Firefox doesn't unpack the extension's file, but merely saves the file to the extensions folder in the Firefox profile folder.

• https://developer.mozilla.org/Updating_extensions_for_Firefox_4.0#XPI_unpacking

To: cor-el Thank You!!!! That was the item causing my problems.

I have no idea how Firefox got installed in "compatibility mode for Windows 98" - I have never seen that before and also did no know about" user agent".

I tried to follow your steps: "You can open the Properties of the Firefox desktop shortcut via the right-click context menu and check the Compatibility" tab" but I got lost. I got to it another way and deselected the tab.

Any idea how compatibility got set up? I would like to know what I might have done wrong.

You're welcome.

We've seen quite a few cases lately where Firefox on Windows XP is running in compatibility mode for Windows 98 (NT 4.10) or Windows (95 NT 4.0), but why this is happening is unclear so far.