Just released a Currency Tracker plugin for Fava: GitHub - Evernight/fava-currency-tracker: Fava (Beancount) plugin for tracking currencies and fetching historical prices using UI
It's meant to be an improved experience over Commodities page in Fava that gets really messy once you have a bunch of different commodities and historical price directives.
Also the plugin gives you an ability to fetch historical prices from internet using bean-price and pricehist libraries without leaving your browser.
I found it weird that I have a monster of a financial tool running on my machine but occasionally still have to go to search and go to external site to look at the historical currency rates in a less-than-optimal interface. Now this is solved, and is good enough at least for me 
The GitHub - andreasgerstmayr/fava-dashboards: Custom Dashboards for Beancount in Fava and GitHub - andreasgerstmayr/fava-portfolio-returns: fava-portfolio-returns shows portfolio returns in Fava plugins were referenced for the technology stack, check these out as well if you haven't yet.