If you go and install an old version of Netscape, you'll see that Thunderbird scans the "C:\Program Files\Netscape\Communicator\Program\Plugins" directory for plugins and lists npswf32.dll as "Shockwave Flash 6.0 r29" under installed plugins. If you rename the file to ...old, the plugin isn't found anymore.

That is the expected behavior. Why do you think that this is a bug ?

Thunderbird shouldn't look in Netscape's directory. It's a different program.

There is no default shared location for installed NPAPI plugins. Scanning in the known directories of other applications using NPAPI plugins is a common way to make the first installation easier for other users. The Netscape days are however over (RIP !) and in many cases you will only find old Plugins with many security holes there. I think it's a good idea to remove the scanning of the NSCP 4x folder. This should be the place with the scanning: http://mxr.mozilla.org/mozilla-central/source/modules/plugin/base/src/nsPluginDirServiceProvider.cpp#250

This might be related to bug 596078.

Resolving old bugs which are likely not relevant any more, since NPAPI plugins are deprecated.