I really appreciate Firefox to have their own certificate DB because it allows to create separate profiles with different trusted certificate sets. The country where I live issued it's own root certificates which I have reasons not to trust in all cases but when accessing government services etc. So I've created a separate Firefox profile where I've installed those malicious certificates, keeping the main Firefox profile clean.

It's mostly historical. They used to suck in the past, but the implementations remain. Nowadays you are better off configuring to use OS certificates if possible.