To use Firefox in the same way as Chromium --app mode (no window borders or other decoration), I create a separate profile called App by running:

firefox -P -no-remote

and then customising that new profile as normal.

Then I create a new directory and file chrome/userChrome.css under the new profile with this useful css that I found here:

* Do not remove the @namespace line -- it's required for correct functioning
@namespace url("");

* Hide tab bar, navigation bar and scrollbars
* !important may be added to force override, but not necessary
#TabsToolbar {visibility: collapse;}
#navigator-toolbox {visibility: collapse;}
#content browser {margin-right: -14px; margin-bottom: -14px;}

All that allows me to run a URI inside a chromeless Firefox window using:

firefox -P App --new-window