Well, the APIs that make sense for various browsers can be shared by those browsers. The APIs that are specific won't be.
For example, Chrome doesn't support sidebars. Opera does. So, Opera constructed a sidebar API. Then came along Mozilla, also wanted to have sidebar, and so they adopted Opera's sidebar API, allowing extension authors to easily support Firefox and Opera.
They can't support Chrome, but it's not like people didn't previously already write sidebar extensions when they could only support one browser at a time.
For example, Chrome doesn't support sidebars. Opera does. So, Opera constructed a sidebar API. Then came along Mozilla, also wanted to have sidebar, and so they adopted Opera's sidebar API, allowing extension authors to easily support Firefox and Opera.
They can't support Chrome, but it's not like people didn't previously already write sidebar extensions when they could only support one browser at a time.