I used WebExtension polyfill[0] when adapting my FF addon to Chrome and admittedly all the intricate differences between APIs still costed me half a day of work.

I managed to have it done with only a few places where I branch on navigator.vendor, but If I wanted to ship different versions to AMO and CWS, I'd make use of something like DefinePlugin[1] for webpack to include/exclude code based on build target.

[0] https://github.com/mozilla/webextension-polyfill/

[1] https://github.com/webpack/docs/wiki/list-of-plugins#definep...