System addon file structure

Hi folks,
I’m a beginner in add-ons, so please keep in mind my maybe stupid questions :slight_smile:
it’s easy to extract standard signed .xpi, because all of them starts with PKZip local file header and are easy to extract via 7zip.
But when I want to extract some system add-on from Firefox installation dir, file format is unknown for 7zip - let say “dirty”. In hex view I see that first four bytes are offset to install.rdf file, followed that PKZip central directory file header.
Why these system .xpi are in different file format and what is the benefit of that?
Also, this system add-ons are without signature, what can be the possible answer, but it just gives me the second question:
If there is a chance that someone can build this “dirty” .xpi and deliver it to installation dir, can he avoid signature requirement? (no, I really don’t want to do that)

Thanks