Plugin API is the magic responsible for Java or Flash in your browser. A plugin once installed on your PC may run in a number of browsers, as all use the same API - currently it is NPAPI.
NPAPI (Netscape Plugin API) is very old and Chrome with Pepper wants replace it.
Mozilla doesn't like Pepper, so at least for now Pepper is a Chrome-specific thing: https://wiki.mozilla.org/NPAPI:Pepper
If companies are people, Google would be someone like Linus, and Mozilla would hang with RMS.
"""Netscape-style plug-ins are supported in Internet Explorer 5.5 Service Pack 1 (SP1) and earlier, but are not supported in Internet Explorer 5.5 Service Pack 2 (SP2) and Internet Explorer 6.""" -- http://support.microsoft.com/kb/306790
I just remember, that there were two versions of Flash - ActiveX for IE and NPAPI for all the others.
It's possible that Mozilla will come around, if Chrome gives them a big enough kick in the pants. Then again, it's equally possible that PNaCL / PPAPI will remain simply an app distribution mechanism for ChromeOS.
You could start at http://src.chromium.org/viewvc/chrome/trunk/src/ppapi/api/