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.
