I'm not sure I understand how your problem happened Scriver. You say that a new Firefox installed itself, but you have the old one still on your system? That's nothing to do with updates then, as far as I know. You'd have to manually run the installer again for that to happen. Also, you can turn off updates in any version of Firefox.
Also, since you said that it's a separate install to your old one, then naturally, it can't access the user settings from your previous browser into the new one. They're different installs. You can only transfer over settings when it's an update of the existing install. The "firefox account" thing is a way to preserve settings even when it's a completely fresh copy of the browser install.
What OS, and what version number is the old one? If the old one is pre-67 then it would be clear why it couldn't update so installed a separate copy. Firefox was entirely re-written for version 67 onwards, so it's not really a compatible install.
My recommendation is to use portable installs wherever possible. Google portable apps and the program you want. Much less hassle, easier to back up, since they store the profiles in the same folder as the application. I have two portable versions of Firefox. One, of the final Version 66 browser (updates turned off just in case), to make use of bulk downloaders and other XUL extensions which are not compatible, and one of the latest Firefox.