It seems they do? Not compoletely polished.

https://docs.firefly-iii.org/en/latest/import/ynab.html

--- EDIT: misread! I don't know about the _features_. They support importing, which doesn't imply supporting the features.

They claim they have budgets, 'categories', and something I wish YNAB had, 'tags'. Tags would be nice so that I could categorize expenses normally, e.g. 'restaurants', but also group them, e.g. 'trip to X'.

I have no idea what YNAB is but yes it is supported:

> You can also import from bunq, from YNAB, using FinTS or simply by uploading CSV files. [1]

[1] https://hub.docker.com/r/jc5x/firefly-iii