I was just stepping through my Options settings after the install of Firefox Portable 66.0.4 and noticed that 66.0.4 had in the 'General' settings enabled the setting "Automatically install updates (recommended)". I never ever allow Firefox Portable to do that, I only ever use the setting "Check for updates but let you choose to install them". So 66.0.4 must have reset that setting to the Mozilla default on install.
Same thing happened in my fully installed Firefox 66.0.4.
So if you don't want Firefox automatically installing updates best go check that setting and put it back to your own preference if it was altered.