As said in the tutorial of Google Chrome extension, NPAPI plugin is supported by Chrome.
1. Supposedly the NPAPI Plugin mentioned here is different from a Firefox Extension, right?
2. If so, is an Firefox Extension supported by Chrome?
3. If I am having a Firefox Extension, which defines an interface "MyInterface". And in the javascript file, how to initialize this MyInterface and how to call its functions?
4. If question 3 is not feasible, could a Firefox Extension be transformed into a NPAPI plugin easily? How?

it isnt, but it really should be, i have a feeling firefox may just adapt to use all of chromes extensions, but not vise versa >.<, chromes extensions are a bit more confining than firefox, thus for them to work, certain security issues would have to be resolved. But with advanced permissions i still think they should