When visible (via devtools.aboutdebugging.showHiddenAddons=true), the system addons and hidden extensions are mixed in the regular extensions category.

It could be nicer to have subcategories for each of them:

> Temporary Extensions

> Extensions

> Hidden Extensions

> System Addons

Although compared to the old about:debugging, since we also have tabs, workers and processes in the same UI, maybe it starts feeling like too many categories and we should rather provide easy ways to filter?

Hidden is an attribute of extensions that are builtin, of which system is a subset. Having a separate hidden extensions section doesn't make sense. I'd just show hidden extensions by default, and do:

temporary
extensions
builtin extensions

The last will catch any hidden extensions and includes system extensions.