Now I did change the CPU - no change, so this is not CPU, most probably not any hardware at all. Booting Windows 10 in safe/minimal mode gets rid of all problems, as much as I can tell without network. My inplace upgrade to 21H1 keeping apps and settings on 19-may-2021 did somethig nasty mixing things up. It is a driver conflict of sort, CPU/Chipset/Management etc. I suppose only new clean install may fix things. Some code affects the CPU on the CPU-cache level, probably with cache overflow at some point, this affects how CPU communicates with RAM and FPU and screws things up, but only Firefox (MS Office not anymore) and some prime number calculations are affected. I might try a full clean re-install with Win11 later.

Last observations: If after PC Win10 boot IPDT64/AIDA64 show FPU/prime-number-test failure Firefox will crash 100%. In Win10 safe-mode+network no problems. PC reboot 1..5 times usually also corrects the issue. If somebody cold have an idea what it could be, I'd really appreciate this.

Problem seems to be solved: Realtek on-board HD Audio disabled in BIOS on ASUS MB. Windows safe-mode disables any audio as well and it was trouble free. On-board audio seems to be having problems only during booting up - it did function well nevertheless (with a microphone).

Additional research points to Realtek/ASUS HD Audio drivers, most probably "Nahimic" service, as a culprit.