Sign in without syncing?

Like many I was confused about the recent message regarding the free VPN which is being rolled out. I have no ICON in Firefox, and whilst I now understand that this is because it is being rolled out gradually, I did initially try a couple of things to try to find it. One of these was to sign in to my account, however, when I did this my browser went crazy. Loads of bookmarks disappeared, as did all the folders that I had these filed in, and loads of very old redundant bookmarks that I had long ago deleted were back. It seems that Firefox synced with 'something', but as I only ever use Firefox on this PC, I don't know what. Also, the browser no longer worked - I could search for anything and I got results, but I got a crash message if I clicked on any of them. Using Edge to search instead, I discovered how to roll back a sync, so I did that and now Firefox is back to normal with all my current bookmarks, history etc.. Obviously I am not inclined to sign in to my account again for fear of this happening again. I assume that when the free VPN finally becomes available to me I will need to sign in. My question then is: is there a way of signing in to my account without initiating a sync of any kind so that my browser stays as it is? (I am actually using Edge to ask this question as I couldn't pose it without signing in and didn't want to risk doing that in Firefox!)