Reproducible builds and open source sounds like a good thing.
I wouldn’t expect the reviewers to deal with every add-ons bespoke snowflake build. Even less so if it requires access to a private module. Mozilla should provide a baseline of how a build is intended to be done, then extensions just have to follow this template. Though yes, you would expect them to have some familiarity with basic stuff like yarn and that the baseline supports a few of the most popular builders.
It seems reasonable that they'd have a requirement there's a single file they'll run, maybe even with a predetermined name like ./build, and that's it.
The developer can then juggle all their dependencies and run make/yarn/npm/etc within that. It's really not different from having a CI build script.
We use a relatively simple build. at the base of it, if you have node and npm, a complete build is as easy as
yarn npm login
yarn --immutable
yarn build
Personally - I don't really find it reasonable to place demands on build tooling for an external company.
I'm assuming you would also find it reasonable for Google to suddenly ship chromium with a requirement that you use "google-pack" for all js builds or they don't run it?
To be entirely blunt, what exactly do you think is going to change when we're already giving them bare JS? It's not like we're shipping a binary blob here, we're literally handing them a zip file with perfectly fine & inspectable javascript inside it.
Further, do you realistically believe that a single low grade QA/Support engineer who can't even install the correct tooling is going to catch malware?
Because I read their matrix chats and I can fucking promise they aren't catching the malware all that fast....