Microsoft supposedly removed support for Netscape-style plug-ins in Internet Explorer version 6.0, and in version 5.5 with Service Pack 2. But it seems they didn't try very hard.
IE5.5sp2 and IE6 still include a file named "plugin.ocx", which is an ActiveX control that can run Netscape plug-ins. The only thing that's been removed is the capability to auto-detect plug-ins and run them at the appropriate time. While I don't know any way to restore that capability, you can re-enable specific plug-ins by associating their MIME type with the ID of Microsoft's plugin.ocx control. To do this, just create one registry item:
[HKCR\MIME\Database\Content Type\your/mime-type]
"CLSID"="{06DD38D3-D187-11CF-A80D-00C04FD74AD8}"
For full-page support, you need to create:
[HKCR\CLSID\{06DD38D3-D187-11CF-A80D-00C04FD74AD8}\EnableFullPage\.your-extension]
(This should be an empty key, not a string value.)
It's probably also necessary to make sure IE's plug-in registry lists your plug-in:
[HKLM\SOFTWARE\Microsoft\Internet Explorer\Plugins\MIME\your/mime-type] "Extension"=".your-extension" [HKLM\SOFTWARE\Microsoft\Internet Explorer\Plugins\Extension\.your-extension] "Location"="path-to-your-plugin.dll"There also may be settings elsewhere in the registry that can interfere with this in some circumstances, such as under
[HKCR\.your-extension]
I have received second-hand reports which suggest that this hack may not work on every computer, but I don't know if that's true or not. Obviously, this is not something that is supported by Microsoft, and they could easily disable it in future versions of IE, should they choose to do so.
I do suggest that you at least not assume this hack will work on the older versions of IE that do support plug-ins.