You don’t need to keep the “META-INF” folder, as we replace all existing certificates.
I just installed version 5.2 of your add-on and it doesn’t show a warning about it being unsigned, so it’s signed just fine.
You don’t need to add the “signed” suffix to your version number. We added this suffix just once to push signed updates to already published versions of add-ons to Firefox users. All approved versions automatically get signed immediately (without the necessity to append the suffix).
So, you’re all set, there’s nothing you need to do.