[Cashtab] Load FIRMA agora page with price in fiat instead of XEC

Summary: For FIRMA, it is more relevant to load the OrderBook in fiat prices than XEC. Do this by default.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D18044