There are often problems with flatpak apps (for me) this is why I prefer snap. But in this case you need sudo apt install xdg-desktop-portal-gtk to be able to upload files and attachments to web pages. Again BUT in this instance it doe not fix the issue. I have installed the native mozilla app on my desktop and it works great. Better than the flatpak or snap packages (also in regards to add ons where I had issues). So, thank you for the info/tutorial - helps a lot. Firefox is still my favorite browser but if Mozilla is concerned about security and functionality - do not take shortcuts for convenience.