Firefox will always update if set to default. Not sure what this has to do with sync? If you use sync on both with the same login it will sync regardless unless you picked the sync options.

My question was not answered. How can I be assured that the unsynced Firefox on the computer that hasn't been used for a while will sync to the Firefox with the current data files and update them instead of my synced current Firefox updating itself to the older Firefox data files on the computer that hasn't been used for awhile?

There still is no response that answers my question posted on 12/172024 as of today, 12-20-2024!

The Daily computer has your current data and when you sign into the old computer, it will sync with the daily computer and not the other way. After they are synced and you add something new on the old computer, it will then sync on the daily and vice versa. see old screenshot I recommend you keep your data on another device in case your daily goes BOOM. I also run most versions of Firefox and Waterfox

I still need exact steps to take to be sure that the daily computer does not try to sync to the unused computer. Right now, the daily computer sync is off. I'm not sure what the sync state is for the unused computer.

So, the steps I need to take I presume is to boot the unused computer and make sure Firefox sync is off and then turn on Firefox sync on the daily computer and then turn on sync on the unused computer? Is this correct?

I have a lot of important Bookmarks so I need to be sure I do it right

Thanks.

Your data should be backed up. There are no steps. You just sign into your account on the old computer and it will sync with the data from the daily. As a test, on the old computer. Download another version of Firefox(Beta, Dev, or Nightly), make sure to install in a different directory than your current installed version, sign into your Mozilla account on the old computer using the different version of Firefox and you'll see the process.