Earlier quoted context omitted.
Similar boat. I release an extension with about 1 million installs across Chrome/Firefox/Edge for work. Firefox (despite being the smallest usage) is utterly insane with regards to process. They demand a reproducible build, but then can't do things like install the right version of yarn (no - npm install -g yarn is not correct, our readme says it in bold like 5 times and provides the exact correct command to install…
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 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.