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 the right version), or follow basic setup steps like "Use this version of node (complete with exact steps to install it and a script to automate that for them)".
God fucking help you if you try to do something completely crazy as a private company like - checks notes - use a private NPM module. Despite providing them with access on a pre-configured account, or offering to give a review account access according to Mozilla "It's too hard to use external accounts during review".
Honestly - having to interact with the browser review team is a BIG reason I no longer recommend Firefox. They're incompetent at best, and I'm fairly convinced they're just milking the google search deal income for as a much as it's worth - I don't think they really want to provide an alternative and secure browser anymore.