After bug 1635718 was patched, the file chooser dialog stopped working on Sway:

(firefox:5789): Gdk-WARNING **: 10:41:57.438: Server is missing xdg_foreign support

(firefox:5789): Gtk-WARNING **: 10:41:57.438: Failed to export handle, could not set transient for

(firefox:5789): Gtk-WARNING **: 10:41:57.440: Can't open portal file chooser: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: No such interface “org.freedesktop.portal.FileChooser” on object at path /org/freedesktop/portal/desktop

Regarding the first message, please see https://github.com/swaywm/sway/issues/3273#issuecomment-445519370

There should be a fallback if xdg_foreign support is missing or at least a way to configure whether to use portal for the file dialog.

Okay, I'll look at it.

With the patch here you need to add widget.use-xdg-desktop-portal boolean pref value at about:config and set it to false.

Thank you, Martin. I tested your patch and it is working.

https://hg.mozilla.org/mozilla-central/rev/348cbf51ddec

Gnome Wayland is my second desktop on Debian Testing. (main is KDE)
The file picker does not show up by default. This pref is inexistent by default. It works after creating widget.use-xdg-desktop-portal and setting it to false. I assume current behavior is not desired.

(In reply to Jan Andre Ikenmeyer [:darkspirit] from comment #7)

Gnome Wayland is my second desktop on Debian Testing. (main is KDE)
The file picker does not show up by default. This pref is inexistent by default. It works after creating widget.use-xdg-desktop-portal and setting it to false. I assume current behavior is not desired.

It should be enabled and working under Gnome/KDE. In KDE you should get KDE native file dialog instead of the Gtk one.
Do you have any error message on console/terminal?

Guys, can you please attach a a wayland log from sway and other non-working environments? Get it by:

WAYLAND_DEBUG=1 MOZ_ENABLE_WAYLAND=1 ./firefox

Thanks.

Gnome Wayland, Debian Testing
WAYLAND_DEBUG=1 MOZ_ENABLE_WAYLAND=1 mozregression --launch 2020-05-25 --pref gfx.webrender.all:true --process-output stdout -a https://interactive-examples.mdn.mozilla.net/pages/tabbed/input-file.html &> filepicker.txt

gnome-core does not depend on https://packages.debian.org/bullseye/xdg-desktop-portal.
Only flatpak would need xdg-desktop-portal. I have confirmed this with debtree gnome-core.

Same with Plasma Wayland on Debian Testing:

0:28.51 INFO: b"(firefox:64147): Gtk-WARNING **: 18:03:17.718: Can't open portal file chooser: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.portal.Desktop was not provided by any .service files"

With Ubuntu 20.04 it's less likely to hit this problem: ubuntu-desktop-minimal depends on xdg-desktop-portal-gtk.
On Debian Testing it is only required by flatpak.

https://hg.mozilla.org/mozilla-central/rev/b28bc5d314b5

On Sway with widget.use-xdg-desktop-portal set to true, after pressing Ctrl+S:

[3619079,071] wl_keyboard@78.key(472964, 229959748, 31, 0)
[3619079,108] wl_keyboard@29.key(472964, 229959748, 31, 0)
[3619331,528] wl_keyboard@78.key(472965, 229960000, 31, 1)
[3619331,565] wl_keyboard@29.key(472965, 229960000, 31, 1)

(firefox:1596780): Gdk-WARNING **: 10:50:20.903: Server is missing xdg_foreign support

(firefox:1596780): Gtk-WARNING **: 10:50:20.903: Failed to export handle, could not set transient for

(firefox:1596780): Gtk-WARNING **: 10:50:20.905: Can't open portal file chooser: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: No such interface “org.freedesktop.portal.FileChooser” on object at path /org/freedesktop/portal/desktop

I do have xdg-desktop-portal installed, because it is a requisite for screen sharing support. Installing xdg-desktop-portal-gtk does not change the warning messages.