How can I launch FireFox with no window frame or tabs / address bar?

Solution 1:

For that, there once was an addon called "Open Chromeless" – but it was removed from AMO. A work-around is described here; I've just checked it and it seems to work. Partly. Cannot get rid of the addressbar, and there's a frame around the window. If you can live with that:

1. Press Shift-F4 to open the Scratchpad

2. In the editor, enter

   window.open("https://superuser.com/", "_blank","width=800,height=600,resizable");
   

   (replace the URL with the one you want to use)

3. Click "Run" on the scratchpad toolbar

Now a new "chrome-less" window should open with the page specified. Resize it to your needs:

^{Screenshot of the "chromeless window" (click to enlarge)}

Right-click next to the addressbar, as shown in the screenshot, lets you disable the address bar as well (untick "Navigation toolbar") – but note that would remove it from your main window as well.

I'm aware this does not fully match your request (it's not launching from the command line, and leaves at least a frame around the window), but it comes as close as I could get to it.

Solution 2:

Wanted to leave this here, it seems to be a barely-documented feature of Firefox called "Single Site Browser":

https://support.mozilla.org/en-US/questions/1292666

Apparently, Firefox disallowed hiding of the location bar unless you launch Firefox with this mode. This SSB mode does still show the window frame (caption with window title, maximise, close buttons, etc.), but all other window chrome is gone.

Now I'm looking for a way to use add-ons in this mode... :(