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."