Framework for browser extensions?

Question

Opera

I had a few questions on the topic "What should be the JS framework for creating browser extensions":

1) Native code and console assembly or as a plugin for the same Eclipse?
[I don't know how possible this is, maybe even a separate program?]

2) What API functions should it contain for convenient programming?
[please mention cross-browser api and personal framework features]

3) What is the usage license?
[preferably short and clear, I still don't understand much about it]

I'm interested in this technology and I want to evaluate the requirements of consumers and the frameworks offered on the Internet. In general, any opinions on a given topic are interesting.