With Falkon 2.1.99 Snap 19, I cannot access the host filesystem. I'm stuck in the container one. This occurs when trying to open a file for upload, when trying to import Firefox bookmarks, or when trying Save as… actions.
Such is life in a confinement system without systems to provide host access https://forum.snapcraft.io/t/xdg-desktop-portal-proof-of-concept-demo/1027 You'll have to do read/save through the snap home and then through dolphin copy bits out of the snap home.
OK. Noted the upstream reason. This would have to be solved before finalizing the goal "we at KDE neon are moving to new packaging format Snaps" stated at https://blog.neon.kde.org/index.php/2017/08/29/great-web-browsing-coming-back-to-kde-with-falkon-new-packaging-formats-coming-to-kde-with-snap/