Anonymous
Thank you @Xecure, I did a quick search on mxlinux forum to see whether it is normal to see firefox policies being overwritten by mxlinux but I did not get a satisfying answer like you gave me above. I know it is an always-rolling distro that has no stable or LTS release like others and so it is pretty reasonable, as you said, to control packages to prevent crashes.
I just suspected that it may be caused by a malicious malware or virus that may have infected my browser to control my browser settings remotely without me noticing it
Your answer helped me clear my doubts, thanks !