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My assumption is that among {NPAPI/Windows, NPAPI/Linux, PPAPI/Windows, PPAPI/Linux}, NPAPI/Linux provides the least value for the work required.

Its more like among {NPAPI/Firefox/Windows, NPAPI/Firefox/Linux, NPAPI/IE/Windows, PPAPI/ALL, etc}, NPAPI/Firefox/Linux provides the least value.

There is no NPAPI/IE/Windows, for what it's worth. It's ActiveX/IE/Windows.