> extension makers will only consider porting to Firefox if it's as little effort as possible. Hence they need to support the same API's
At least the last time I developed a browser extension, this was not the case. I found that what was considered the WebExtension API was not 100% portable across browsers, and I had to rewrite parts of my code between Chrome and Firefox because Chrome had many extra APIs that Firefox lacked.
If you compare with attempting to share code between XUL addons and Chrome extensions, you'll notice that things have gotten muuuuuuuuch simpler, though.
"As little effort as possible" does not mean that it should be perfectly identical - sometimes that's just not an option. They're striving to be as close as possible though, and this is part of that.
At least the last time I developed a browser extension, this was not the case. I found that what was considered the WebExtension API was not 100% portable across browsers, and I had to rewrite parts of my code between Chrome and Firefox because Chrome had many extra APIs that Firefox lacked.