Having previously used F3, there is a separate importer app that will load data from a few different formats and import it in. The kind of things you can export from your bank web site. I went so far as to script selenium to download the data from my bank, but it would constantly break because my bank did some silly things with cookies. There's not much support for any sort of automatic importing directly from your bank, which was the reason I stopped using it and went to Actual Budget, which supports SimpleFIN linking to tons and tons of different accounts. I remember there were a few different threads to get similar support in Firefly but they didn't seem to get much traction.

F3 is very cool and powerful, though it's also strongly opinionated. I used it for about a year and ultimately it didn't click for, but it really hits the spot for a lot of people.

Afaik the maintainer has a semi-firm position on the tool being intended for the manual import of transactions.

There is, however, the third party firefly-fints-importer[1] which I'm running without any issues.

[1] https://github.com/bnw/firefly-iii-fints-importer