Is there a way to do this with user CSS on Firefox?

The content has to have full opacity. So setting opacity through the compositor or the opacity CSS property does not count.

Setting a background-color with some alpha on userContent.css technically works but because the browser itself is fully opaque, doesn't show the background. Trying to set background-related properties on #browser on userChrome.css doesn't have an effect. If I can make only the background of #browser transparent, this'll work. But I don't know if that's possible.

edit: it is possible, with lots of caveats. Site-specific fixes will be required, popups are hard to read. If you have dark wallpapers enable dark reader to make site content readable on a dark background.

1- set browser.tabs.allow_transparent_browser to true

2- in userChrome.css add #main-window, #tabbrowser-tabpanels{ background: transparent !important; }

3- userContent.css:

*{
  background-color: transparent !important;
}
/* if you don't want full transparency */
html:root{
  background-color: #00000080 !important;
}