[creesch]

Technically correct, but it is a bit more complex. The original web extension API is based on the chrome extension API. So most (there are some annoying exceptions at times) of the chrome extension API calls also work with very little adjustment on firefox. It becomes even easier when you use mozilla's polyfill library https://github.com/mozilla/webextension-polyfill

Then you can just target the promise based webextension syntax and as long as you still stick to the calls also available in chrome your extension works with very little effort in both browsers.

Safari is a different story which basically amounts to Apple being Apple and sort of supporting webextensions but in such a roundabout way that it is barely worth it for the majority of extension devs.