TL:DR; because Firefox adopted the Web Extensions format and even links chrome.* to the same browser.* api, so all you need to do is change a couple of things in your manifest.json and you're good to go.
What they don't say how long it takes for Firefox to approve your extension. I submitted an extension and I it took almost 3 months to get approved. I got put in a queue of about 300 at the time and the queue moved very slow. I think this is because an actual person is reviewing the extension and not some automated tool (I assume, maybe that live person is using an automated tool and then hand-checking the results)
What they don't say how long it takes for Firefox to approve your extension. I submitted an extension and I it took almost 3 months to get approved. I got put in a queue of about 300 at the time and the queue moved very slow. I think this is because an actual person is reviewing the extension and not some automated tool (I assume, maybe that live person is using an automated tool and then hand-checking the results)