This is not just ridiculous, this is getting beyond ridiculous. Guest additions 5.2.2 in Ubuntu 18.04 VM give me the same problems. I can't run Firefox, mate-tweak or even glxinfo with 3D acceleration enabled. However, it gets even worse after I turn it off: the system won't even boot properly.
virtualbox-guest-* packages is a good workaround, it allows kernel's own vboxvideo and vboxguest modules to load. Though, as mentioned above, they don't have host/guest clipboard support.