userChrome file could resolve the UI thing, use something to have chromium interface
for performance, all that i could see is that profiles are ignored, both on chromium n firefox, i use a lot of profiles to minimize addons impact on performance, and with a lowend computer i always got a smooth xp on firefox
it has something like '-no-remote -p name' param on cmd that you can do it seamless like chromium, or u can use the fork of the drop official pwa firefox support, it could be better, i know n i get it, but if u just use chromium base for it, than i got u covered