Is there a command line flag that opens the url in a chromeless window for Firefox, so that it works like a PWA?

I haven't messed with Firefox's PWA feature/taskbar-tabs[1] but it seems like under the hood it's just using shortcuts so you're probably looking for -taskbar-tab

This redditor looks like he's doing close to what you're asking with the caveat of needing an entry in taskbartabs.json to use -taskbar-tab (and maybe Windows OS only) [2]

I don't think it's completely chromeless but you can probably hide everything via custom CSS:

ctrl+shift+i (dev tools) -> f1 (settings)

check advanced settings:

  enable browser chrome and add-on debugging  

  enable remote debugging  

ctrl+shift+alt+i (browser dev tools)

Find element IDs or class names and try out CSS rules to your heart's content.

Set toolkit.legacyUserProfileCustomizations.stylesheets = true and put your changes in Firefox/Profiles/$profilename/chrome/userChrome.css.

Remember to turn off the advanced settings when you're done.

[1] https://firefox-source-docs.mozilla.org/browser/components/t...

[2] https://www.reddit.com/r/firefox/comments/1ljidwp/progressiv...

Cannot get this to work with Win11, whose taskbar won't accept the same program pinned twice. Even not, after creating two shortcuts.

For Win 10, I set this up a while ago so my memory is a bit fuzzy but the key was the `taskbar.grouping.useprofile true` setting. I think after that's set Firefox and Windows did some AUMID shenanigans on profile launch.

I just made a new profile as a test and this is what I did:

  ...firefox.exe" -P -> create a new profile (ie. "newprofilename")
  
   about:config set `taskbar.grouping.useprofile = true` (when I originally did this for many profiles I believe I copied prefs.js that already had it set)  

   close and reopen that profile instance. I used ...firefox.exe" -P "newprofilename" but any method of launching the profile should work. It should now be in a separate taskbar group.  

  Pin that new group to taskbar. Also modify its shortcut target to add -P "newprofilename"  

  Now you're done.  

Normally I also renamed the pinned shortcut to something sane and then I changed the icon. I took the normal Firefox icon (I think w/ GIMP) and just messed with the colors via saturation or something so it was easy to tell the difference. I remember changing the shortcut icon had some headache but I sadly didn't write notes.

Also, I didn't set grouping.useprofile on all my profiles, just the profiles I wanted separately pinned on my taskbar. My default profile is pinned normally without grouping.useprofile set.

Weird this works for Edge with two different profiles.

I do this on Linux (Mint) and it works as expected.

>from terminal [opening URLs in instances using profile paths] works exactly as expected regardless of how many profile instances are currently running or their state.

That's interesting. It didn't work a couple of years ago, perhaps it's been fixed.

Still leaves that initial GUI profile setup.

You don't need GUI to create profiles.

   ./firefox -CreateProfile "profile-name /home/user/.mozilla/firefox/profile-path/"

This is a good catch and does indeed work.

  -CreateProfile "profile-name"

also works and will use the default profile path.

Keep it mind, though, if you don't specify path, the profile directory name would be:

    random_string + '.' + "profile-name"

That could prove inconvenient for navigation.

If you want predictable, non-random directory names, you'll have to specify it.

Useful. This is not documented in the manfile or command help text, at least for the ESR release.