Is there really no viable web browser, or any kind of insane-ish workaround hack, to display normal modern websites like Youtube or Facebook?
I think you're missing the point of the whole LibreELEC/Kodi application.
If you want to do your social media during a video, use your smartphone.
mean, in principle the box is running Linux and its just a matter of cross-compiling enough stuff to make Firefox run
Well, if it that's easy... Why didn't we think of that?!?
I can't really express how important a browser is to any kind of modern media device. We all should know this?
I call social media one of 21th century's addictions.
If nothing really works yet, then please which project in this direction is actively developed and promising and where to find it and help develop?
Raspbian has some Chrome/Chromium-type web browser.