There is an approval process to use the allow-sandboxing and it does require vetting because it gives your snap more privileged access over the machine than most other snaps.
But security is multi-facetted. Installing your snap means I give you a lot of trust anyway; but in the case of a browser; that trust is absolutely massive. The snap/browser will by default ask to save the credit card details of the user, passwords, etc. The snap sandboxing IMO means nothing in that instance, the sheer nature of being a general purpose web browser is a huge responsibility and there’s a lot of damage the browser can do to itself. I’d go as far as to say I’d personally prefer a web browser was run in classic confinement where the internal sandbox worked, than in strict where it doesn’t. The snap sandbox protects the machine from the snap. It doesn’t protect the snap against itself.
I do expect if you were to ask for a review for the interface you’d likely be declined it due to the vetting process, if so then that leaves us in the current situation where the snap is already collecting a massive amount of privileged information and exposed to a huge amount of evolving threats with half it’s security model disabled, being direct, I’d say this either needs to be handed up to Librewolf to fit within the current policies or removed for being a liability.
(I don’t want this feedback to come off negative, I do appreciate the effort and it’s nice seeing you grow in your snapping journey. I wouldn’t trust myself with a browser snap either, it’s a lot of potential risk to have in the hands of one person!)