fix: add `build-firefox` to support background script

[hakadao]

Description

The original build command doesn't support the Firefox background script

Linked Issues

Additional context

[antfu]

Not so sure as it duplicates the scripts, making it harder to maintain. Maybe we could document it instead, or have it in pack:firefox?

[hakadao]

Perhaps this is the right thing to do, but I currently don't know how to make the changes 😕 I'm sorry for taking so long reply🙏