Their product appears to be mostly an extension, with a small amount of bespoke browser code which necessitated a fork. I believe they are saying the delta is smaller with a Firefox base because there are more pre-existing extension APIs they can reuse (and rely on someone else to maintain).

I don't understand why this sort of product exists though. As a user, whether or not I'm aware of the difference, I don't want the additional memory overhead of running a separate browser, the separate cookie jar, the delayed updates, the additional bugs... the better approach would obviously be to work with FF developers to add the extra extension APIs they need; I can only assume they took that path but got rejected. If you're a Firefox user, ie someone who inherently places their trust in the security and privacy decisions of Mozilla, that's probably a red flag.