I wonder if it is the Intel gen 11 CPU and newer and 6.3.6 CPU with ibt enabled are the Firefox issue as my gen 10 does not have any issue with Firefox.
Indirect Branching has to be set in a Table and passed to the kernel for permission to work if ibt is enabled - virtualbox will not run unless ibt=no is set in the kernel. I wonder if that is the Firefox issue as well. Try booting with ibt=no on bootup.