There's no WebExtension API for accessing or managing the Firefox internal whitelist so what you are asking is impossible (at least at the moment).

However, I recommend you to use just Cookiebro. First allow all cookies in Firefox settings. Then in Cookiebro "Enable blacklist filtering" in Options, enter one line to blacklist: *.* and then whitelist the sites you want either by using the Cookiebro whitelist or when needed using the Cookiebro extension icon action. That will then completely block all cookies except the ones you whitelist.

You don't need the Firefox internal whitelist system at all and as you said, it's very cumbersome to use.