For what it's worth, my update procedure, done while I"m on line, is:
- Copy new version from .dmg.
- Paste it over my existing earlier version (which is stored in my data partition).
- Launch new version.
- Click to OK download.
and that procedure perpetuated my prefs, etc.
I added another edit to my previous one.
I don't understand your update procedure.
• What does it mean to copy new version from .dmg? [I just open the Firefox 67.0.dmg file (as I've always done previously without any weirdness intervening).]
• Your step #2 is completely opaque (incomprehensible) to me.
• Steps #3 and #4 likewise.
Given your own concerns about the procedure, I doubt if I could be persuaded to follow such (even if I understood it).