And yet the GrapheneOS people recommend their own "Vanadium" hardened version of Chromium instead, for reasons I don't understand.

It is explained here https://grapheneos.org/usage#web-browsing

They don't explicitly mention Firefox but:

"Chromium-based browsers like Vanadium provide the strongest sandbox implementation, leagues ahead of the alternatives. "

and

"Chromium has decent exploit mitigations, unlike the available alternatives. "

Since I myself lack the knowledge and skills to judge this, I have to trust the word of the developers.

Edit, correction. They do mention Firefox

"Avoid Gecko-based browsers like Firefox as they're currently much more vulnerable to exploitation and inherently add a huge amount of attack surface."

And the complete inability to block ads is just a non issue. Ok.

vanadium has adblocking

Really? Didn't seem to to me; I just installed latest GrapheneOS on my pixel 9 this weekend, and Vanadium definitely let Google Adsense crap all over many sites I visited.

Is there a way to enable effective ad blocking that I missed?

I'm not sure I think it's enabled by default. look in the settings to be sure. works pretty well on my grapheneos pixel 8

Can you show us where the adblocking is? I just went through all my vanadium settings and found nothing.