Firefox

Well, what they were trying to say, is that Firefox's extension API is essentially a superset of the Chrome extension API. If an extension works with Chrome's limited API, then it should be almost trivial to port it to Firefox.

But yes, this is not a guarantee that the same extensions are available.
Some extension devs might not care to support both browsers, just because it may require more work for testing and releasing.
In particular, Mozilla also does not allow extensions to make web requests, unless this is part of the core functionality, so if extensions finance themselves via ads or tracking, they likely won't make more money from releasing to Firefox.