Chromeless Firefox

userChrome.css

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
#PersonalToolbar,
#BookmarksToolbar,
#MenuToolbar,
#nav-bar{ display: none !important; }
    
#tabbrowser-tabs, #tabbrowser-tabs > .tabbrowser-arrowscrollbox {
   min-height: 0 !important;
}
#tabbrowser-tabs tab[first-visible-tab="true"][last-visible-tab="true"] {
   visibility: collapse;
}
#tabbrowser-tabs .tabs-newtab-button {
   visibility: collapse !important;
}
#tabbrowser-tabs tab {
   min-height: var(--tab-min-height)
}

1. Close firefox

2. Run firefox -P
   (If you're running windows, you have to put the full path to firefox.exe instead of just firefox)

3. Create a new profile and name it "app"

4. Select the "Default" profile, check "Use the selected profile..." and click on "Start Firefox"
   https://i.imgur.com/ckJ6o1T.png

5. Open about:profiles and under "Profile: app" > "Root Directory" click on "Open Directory"
   https://i.imgur.com/rRJ0xoQ.png

6. Create a folder called chrome and inside a file called userChrome.css. Inside that file, paste the contents of this Gist

7. Run
   firefox -P app --new-instance http://www.reddit.com
   (or full path to firefox.exe)
   Result: https://i.imgur.com/AdpGTyQ.png

8. Create desktop/menu shortcuts to your favorite webapps

9. Profit!

Credits to https://askubuntu.com/a/487954/593872 and https://gist.github.com/BenoitAverty/af633ee20e27f48f9ba7178451432206

Hi! I've found that for Firefox 68, I need to change the following line
#nav-bar{ display: none !important; }
into
#nav-bar{ visibility: collapse !important; }
in order to make Firefox load the page, instead of a pure empty page.
I am not sure what have been changed in the browser part, I guess it is a init sequence change for Firefox?

FireFox 69 needs to additional enable
about:config
toolkit.legacyUserProfileCustomizations.stylesheets -> true
And if nav-bar not visible the site is not visible either :/