Try to specify a clean profile dedicated for this screenshot, create the profile with the profile manager and remove all files in it.

<path_to_firefox> --screenshot ~/Downloads/test-sumo.png https://support.mozilla.org/en-US/questions/firefox -P test-ss -no-remote

New user, new profile was created specially to report this problem. Now it hangs with this:

firefox --screenshot ~/Downloads/test-sumo.png https://support.mozilla.org/en-US/questions/firefox -P test-ss -no-remote

• • • You are running in headless mode.

Gtk-Message: 03:32:35.322: Not loading module "atk-bridge": The functionality is provided by GTK natively. Please try to not load it. [GFX1-]: RenderCompositorSWGL failed mapping default framebuffer, no dt

You should only use this profile for headless mode as I've noticed before that using this profile for a regular session makes it stop working.

• /questions/1256647 Headless Firefox doesn't work

But it just hangs, no response, no screenshot, no return to command prompt, as with other profiles.

Ok, with this command: firefox --screenshot ~/Downloads/test-sumo.png https://support.mozilla.org/en-US/questions/firefox -P test -no-remote

i finally get my screenshot in ~/Downloads folder. 

But... is this the way it should work? Could you explain or point me to place were i can eliminate my lack of knowledge about how it should work?

For ~/Downloads i have such rights:

ls -la ~/Downloads/ total 1008 drwxr-xr-x 2 test test 4096 May 10 15:03 . drwxr-x--- 19 test test 4096 May 10 15:04 .. -rw-rw-r-- 1 test test 1022906 May 10 15:05 test-sumo.png

For newly created directory test there i have such: ls -la test/ total 8 drwxrwxr-x 2 test test 4096 May 10 15:04 . drwxr-x--- 19 test test 4096 May 10 15:04 ..

As far as i can see the difference in just group rights. Or headless mode works ONLY with some special profile but for common profile it's impossible? Why it is possible to create screenshot for Downloads/ folder while impossible for test/ folder?

I do not know what difference(s) in a profile initialized via starting in headless mode is causing this stop working when you use this profile with a normal session. You can compare two profiles to see whether some file and folder aren't present.