Created new profiles for ungoogled-chromium, Firefox and Pale Moon with separate data and cache directories. Then I contained them with new firejail profiles that don’t have internet access but can see localhost (my usual profiles don’t have access to localhost and the local network). I styled Firefox and Pale Moon with
$XDG_CONFIG_HOME/gtk-3.0/gtk.css to have dimensions as close to ungoogled-chromium as possible (I can’t easily style Chromium, so this was the best option).
Now it’s only a matter of setting the dev console to the exactly same width in the three browsers and I can easily cycle between them to check rendering quirks.