Windows Firefox does not find my plugin

I'm building a skeleton NPAPI plugin to test specific functionality and I cannot get Firefox to see it.  I'm on a 64-bit Windows 7 machine.

I have added:

HKLM\SOFTWARE\Wow6432Node\MozillaPlugins\PluginBug
  (Default)   REG_SZ (value not set)
  Description REG_SZ "Test"
  Path        REG_SZ <path-to-my-plugin>\npPluginBug.dll
  ProductName REG_SZ PluginBug
  Version     REG_SZ 1.0.1
  Vendor      REG_SZ Me
HKLM\SOFTWARE\Wow6432Node\MozillaPlugins\PluginBug\MimeTypes\application/vnd.pluginbug

... it doesn't appear in Add-ons Manager, nor in "about:plugins".

Using Process Monitor, I compared how firefox looked for all plugins; for each plugin (including mine) firefox queried the directory containing it to get a directory listing.  For each (including mine), the listing included the plugin.

But for each of the others, after the QueryDirectory finished, firefox called "CreateFile" with Open/Read... but for mine, after the QueryDirectory finished, it called "QueryOpen" (result SUCCESS).

Why this difference?

I've verified with CFF Explorer that I'm exporting NP_Initialize, NP_GetEntryPoints and NP_Shutdown.

AFAIK nothing else has my plugin open.

I deleted pluginreg.dat and tried again, with no success.

I restarted... same.

First, assuming I spelled everything correctly, is there something I missed?  I'm typing this in manually so might have fat-fingered something in this email.

Thanks,
Rudi