Another disadvantage of using Firefox in a flatpak (compared with either a deb package or a snap) is that because of the file access confinement, Firefox will not be able to remember the save folders for downloaded files, even if you have set up browser.download.lastDir.savePerSite – and I quite like this functionality.

To workaround the issue, open your Flatpak Permissions management application (on Plasma just search for “flatpak” and you’ll find it, or locate it in System Settings -> Applications) and add a “File System Access” entry – with “read/write” permission – to the top directory where you want Firefox to be able to remember folders – I just use /home/odeda/Documents: Firefox has access to write into all of my documents and projects and stuff (basically almost all of my files), but can’t access other applications’ configuration or security keys or stuff like that.

Security is always a trade-off
¯\_(ツ)_/¯