Chromium/Chrome web browser apparently has a built-in support for opening a special window for Google Keep, WhatsApp, etc.
For example, the command chromium-browser --app=https://web.whatsapp.com/ effectively presents you with a nice, neat web-based desktop client for WhatsApp (with no tab bar, bookmark bar, etc.)
How can we achieve the same in Firefox which doesn't have a similar command option/parameter?
1) Make sure that you have ./chrome/userChrome.css under your user mozilla firefox profile directory (e.g. ~/.mozilla/firefox/*profile*/), and that it includes this line to suppress the navigation bar:
#main-window[title^="WhatsApp"] #nav-bar { visibility: collapse !important; }
2) Create an html file (e.g. ~/.bin/WhatsAppWeb) with the following contents to open WhatsApp Web in a popup window instead of a regular window:
<!DOCTYPE html>
<html lang="en-us">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>WhatsApp Web Launcher</title>
<script type="text/javascript">
window.open('https://web.whatsapp.com/','myWindow','width=670,height=900,top=0,left=1250');
</script>
</head>
<body>
</body>
</html>
3) Enter the command firefox -new-window "~/.bin/WhatsAppWeb" in a terminal window, and allow local file ~/.bin/WhatsAppWeb to open popup window from now on by saving this preference, and then close the firefox window(s).
4) Finally, whenever you want to start WhatsApp Web, run these commands - e.g. from terminal or a .desktop file, via bash script, etc.:
firefox -new-window "~/.bin/WhatsAppWeb" &
sleep 5s
wmctrl -ic "$(wmctrl -lp | grep "$(pgrep firefox)" | grep -v 'WhatsApp - Mozilla Firefox' | tail -1 | awk '{ print $1 }')"
In this last step, a WhatsApp Web popup window is launched from a new blank Firefox window, and then that blank window is closed after 5 seconds (which might need to be adjusted according to different conditions), thereby effectively opening a special Firefox window for WhatsApp Web only.
Note: This solution has been found thanks to Ginbread Man here: http://forums.mozillazine.org/viewtopic.php?f=38&t=3035313&p=14782570#p14782570